Solar design tips, sales advice, and industry insights from the premier solar design software platform

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Solar Primer | Solar design tips, sales advice, and industry insights from the premier solar design software platform

As a solar professional, it’s important to be able to explain the process of how a solar photovoltaic system produces electricity. This process seems mysterious to many and misconceptions abound among those unfamiliar with solar energy. In this article, we get back to basics with an overview of how solar installations provide electricity and how the process works for the customer.

How Does a Solar Photovoltaic System Work?

Solar panels convert the energy of photons (light particles) into electricity (as we discuss in The Beginner's Guide to Solar Energy). This process is called the photovoltaic effect. When a photon hits a photovoltaic (PV) device, its energy is transferred from the photon to the local electrons in the material. These excited electrons begin to flow, producing an electric current.

Solar cells (within solar panels) produce direct current (DC) electricity, which is typically converted to alternating current (AC) electricity by an inverter. This allows it to be sent back to the electric grid, which operates with AC electricity, as well as used to power appliances in the customer’s home (or commercial building, in the case of commercial solar installations). In summary, the process of how solar panels works involves three primary steps:

1. Solar cells within solar panels absorb light from the sun, which causes electric current to begin flowing.
2. An inverter converts DC electricity to AC electricity.
3. This electricity is used to supply current energy demands in the customer’s building and excess electricity beyond what the customer can use is exported to the grid.

## What Happens with the Energy a PV System Produces?

Most solar customers in the U.S. have grid-connected solar installations. Their home is connected to the electric grid, which allows them to use energy supplied by their electric utility when they need more than their solar installation is producing (such as on a rainy day or at night). It also means that whenever their PV system is producing more than they need, that excess energy can be sent to the grid for others to use.

A policy called net metering, common throughout the U.S., compensates solar customers for this excess energy, so that they can offset the cost of future electricity they use from the grid.

Net metering has played a significant role in making solar energy cost-effective. However, around the country, we are beginning to see some changes in how utility companies implement net metering, many of which reduce the value that solar customers receive from their solar installations.

Feed-in tariffs are another way of compensating solar customers for the electricity they send to the grid in some areas.

## What are the Parts of a Photovoltaic System?

A simple PV system contains two basic types of components:

• Solar Modules: Solar modules contain solar cells that convert sunlight into electricity.
• Inverter(s): An inverter converts DC current to AC current. It can also perform other functions that are beneficial to the electricity grid (see our article on smart inverters, which are now required in California).

Diagram of a simple PV system. Source: Aurora Solar.

It is common practice to refer to all components of a PV system besides the modules as balance of system (BOS) components. Examples of BOS components include inverters, disconnects, racking, and wiring.

Of course, this is just a basic overview of the parts of a solar installation and how they fit together. Explore some of our related articles for a deeper dive into the ways that solar panels and inverters can be wired together (stringing) and some alternatives to traditional inverters, known as module-level power electronics (MLPE)

## What Factors Affect Solar PV System Efficiency

It is important to note that the process of producing electricity from solar energy is not 100% efficient. Environmental factors—such as temperature, soiling, and shading—as well as losses in the electrical components, can affect the efficiency of a PV system (for a deep dive on these losses see our PV System Losses Series). Typical loss categories include:

• Temperature: Solar panel efficiency varies with temperature. High temperatures have a negative impact on performance.
• Soiling: Material that accumulates on the surface of PV panels can block light from reaching the solar cells, thereby reducing the generated power. The power loss due to soiling is highly variable, depending on the type of soiling (such as dust or snow), and how frequently the panel is cleaned.

Soiling, such as dust, on PV modules reduces power output.

• Shading: Shading is the obstruction of irradiance due to trees, buildings, terrain, and other objects in the environment. The effect of shading on the power output of a solar installation is highly variable. (To learn more about  the causes and consequences of shading, this article and this section of our PV system losses series are great resources.)
• Wiring and connections: Resistance in the electrical connections of a solar installation typically results in energy losses of a few percent.
• Mismatch: Due to manufacturing variations, modules of the same type can have slightly different electrical characteristics. This mismatch between modules can lead to a performance loss.
• Inverter Efficiency: Converting DC into AC current via an inverter is typically around 96-97% efficient. Inverters typically have higher efficiency when the DC input power is high. The conversion efficiency takes a big hit when the input power is much less than the inverter's rated power.
• Age: Solar panels produce less energy the older they get. Typically the decrease in performance is assumed to be around 0.5% per year.
Term Typical Value
Temperature -0.5%/°C above 25°C
Inverter Efficiency 96.5%
Mismatch 98%
Wiring/Connections 98%
Soiling 95% (highly variable)
Age -0.5%/year
Typical solar efficiency values for different PV system loss types.

The above factors are combined in a coefficient called the system derate factor to represent the overall losses of a solar installation. For instance, PVWatts, an NREL supported PV system energy production calculator, uses a default system derate factor of 86%.

However, depending on the system design or environmental conditions, this value can be higher or lower. Advanced solar design software like Aurora can ensure that you accurately determine PV system losses and how much energy your PV system will produce.

Solar panel (module) efficiency denotes what portion of irradiance a module converts into electricity under standard test conditions (STC; irradiance of 1000W/m2, ambient temperature of 25°C). As a general rule of thumb, you can estimate a PV system’s efficiency in converting irradiance into electricity (under STC) using the following formula:

$$\text{Overall System Efficiency} = \text{Module Efficiency} \times \text{Derate Factor}$$

It is important to note that these are merely back-of-the-envelope calculations. To get a comprehensive energy production analysis, you need a software application, such as Aurora, that incorporates all of a PV system’s environmental, mechanical, and electrical characteristics.

### About Solar PV Education 101

How a Photovoltaic System Produces Electricity is part of Solar PV Education 101, a six-article series that serves as an introductory primer on the fundamentals of solar PV for beginners.

Editor's Note: This article was originally published on October 7, 2016. It was updated in December 2019 for freshness, accuracy, and comprehensiveness.

In the solar industry, producing electricity is our bread and butter. That means it’s important for solar professionals to have a strong grasp of electricity fundamentals.

If you’re new to solar, there’s a lot to learn so in today’s article we cover a key topic to understand about electricity: the difference between two types of electric current—alternating current (AC) and direct current (DC). Both are involved in a solar PV system.

## The Basics: How Do Alternating Current (AC) and Direct Current Differ?

As we explain in our primer on solar panel stringing, current is the rate of flow of electric charge (i.e. the flow of electrons).

There are two forms that electric current can take: alternating or direct. Direct current always flows in the same direction. Meanwhile, alternating current—as you might expect from the name, changes direction frequently (though the back-and-forth motion of the electrons still conveys energy to the end device).

“A simple way to visualize the difference is that, when graphed, a DC current looks like a flat line, whereas the flow of AC on a graph makes a sinusoid or wave-like pattern,” says Karl K. Berggren, professor of electrical engineering at MIT.

## Electricity History: The Fight Between AC and DC

When the use of electric power was first being developed, it was unclear whether AC or DC would become the dominant way in which electricity was supplied. Two famous pioneers of electricity—Thomas Edison and Nikola Tesla—each advanced one of these options.

Tesla had patented AC, while Edison had patents on DC. Despite a smear campaign by Edison to discredit AC as dangerous (in which he went so far as to publicly electrocute animals!), AC won out in the long run. AC gained predominance because it was easier for power companies to transmit AC power over long distances.

## Where Are AC and DC Power Used?

Another important thing to understand about AC vs. DC power is what kinds of devices and applications use each.

A solar panel produces direct current; the sun on the panels stimulates the flow of electrons, creating current. Because of these electrons flow in the same direction, the current is direct. Similarly, batteries use direct current; they have a positive and negative terminal and current always flows in the same direction between those points. In contrast, the U.S. electric grid and the power flowing into your home are AC. As a result, most plug-in home appliances run on AC power.

It is for this reason that solar PV systems include inverters! The inverter converts DC energy into AC energy so it can be used in the home or sent back to the electric grid (in addition to some other functions).

You might also be surprised to learn that many of the electronics you use—like your laptop and cell phone—run on DC and have their own inverters as well. The power adapter that is part of the charger for these devices is in fact a form of inverter that converts the AC grid supply to DC power that can be used by the device.

## What Are AC Solar Panels?

As we discussed above, traditional solar panels produce DC energy. That energy is then converted to AC power by the inverter. This is the case whether your PV system includes a string inverter (which converts energy from one or multiple strings of solar panels) or microinverters (which convert it for individual, or in some cases a few, solar panels).

You may have heard of AC solar panels, however. If solar panels inherently produce DC, you may be wondering what these are. AC panels are simply solar panels that have microinverters integrated into them. System design with AC panels is the same as designing a system with microinverters, except that the installer doesn’t need to buy and attach the microinverters.

Understanding the differences between AC vs. DC is important knowledge in the solar industry. Not only is it essential to understanding how a solar array works and how it is designed, this knowledge can also help you educate customers—one way to build trust during the sales process.

What other topics would you find helpful to understand? Let us know in the comments below! The Aurora Blog seeks to provide educational updates for solar contractors and we’re always open to new topic ideas based on needs in the industry!

Topics: solar energy, Solar Primer

Solar panel wiring (aka stringing), and how to string solar panels together, is a fundamental topic for any solar installer. You need to understand how different stringing configurations impact the voltage, current, and power of a solar array. This makes it possible to select an appropriate inverter for the array and make sure that the system will function effectively.

The stakes are high. If the voltage of your array exceeds the inverter’s maximum, production will be limited by what the inverter can output (and depending on the extent, the inverter’s lifetime may be reduced) . If the array voltage is too low for the inverter you’ve chosen, the system will also underproduce because the inverter will not operate until its “start voltage” has been reached. This can also happen if you fail to account for how shade will affect system voltage throughout the day.

Thankfully, modern solar software can manage this complexity for you. For instance, Aurora will automatically advise you on whether your string lengths are acceptable, or even string the system for you. However, as a solar professional, it’s still important to have an understanding of the rules that guide string sizing.

In this article, we review the basic principles of stringing in systems with a string inverter and how to determine how many solar panels to have in a string. We also review different stringing options such as connecting solar panels in series and connecting solar panels in parallel.

Solar panel wiring is a complicated topic and we won’t delve into all of the details in this article, but whether you’re new to the industry and just learning the principles of solar design, or looking for a refresher, we hope this primer provides a helpful overview of some of the key concepts.

## Key Electrical Terms to Understand for Solar Panel Wiring

In order to understand the rules of solar panel wiring, it is necessary to understand a few key electrical terms—particularly voltage, current, and power—and how they relate to each other.

To understand these concepts, a helpful analogy is to think of electricity like water in a tank. To expand the analogy, having a higher water level is like having a higher voltage - there is more potential for something to happen (current or water flow), as illustrated below.

### What Is Voltage?

Voltage, abbreviated as V and measured in volts, is defined as the difference in electrical charge between two points in a circuit. It is this difference in charge that causes electricity to flow. Voltage is a measure of potential energy, or the potential amount of energy that can be released.

In a solar array, the voltage is affected by a number of factors. First is the amount of sunlight (irradiance) on the array. As you might assume, the more irradiance on the panels, the higher the voltage will be.

Temperature also affects voltage. As the temperature increases, it reduces the amount of energy a panel produces (see our discussion of Temperature Coefficients for a more detailed discussion of this). On a cold sunny day, the voltage of a solar array may be much higher than normal, while on a very hot day, the voltage may be significantly reduced.

### What Is Current?

Electric current (represented as “I” in equations) is defined as the rate at which charge is flowing. In our example above, the water flowing through the pipe out of the tank is comparable to the current in an electrical circuit. Electric current is measured in amps (short for amperes).

### What is Electric Power?

Power (P) is the rate at which energy is transferred. It is equivalent to voltage times current (V*I = P) and is measured in Watts (W). In solar PV systems, an important function of the inverter—in addition to converting DC power from the solar array to AC power for use in the home and on the grid—is to maximize the power output of the array by varying the current and voltage.

For a more technical explanation of how current, voltage, and power interact within the context of a solar PV system, check out our article on Maximum Power Point Tracking (MPPT). In it, we discuss current-voltage (IV) curves (charts which show how the panel output current varies with panel output voltage), and power-voltage curves (which show how panel output power varies with panel output voltage). These curves offer insight into the voltage and current combination(s) at which power output is maximized.

## Basic Concepts of Solar Panel Wiring (aka Stringing)

To have a functional solar PV system, you need to wire the panels together to create an electrical circuit through which current will flow, and you also need to wire the panels to the inverter that will convert the DC power produced by the panels to AC power that can be used in your home and sent to the grid. In the solar industry. This is typically referred to as “stringing” and each series of panels connected together is referred to as a string.

In this article, we’ll be focusing on string inverter (as opposed to microinverters). Each string inverter has a range of voltages at which it can operate.

### Series vs. Parallel Stringing

There are multiple ways to approach solar panel wiring. One of the key differences to understand is stringing solar panels in series versus stringing solar panels in parallel. These different stringing configurations have different effects on the electrical current and voltage in the circuit.

#### Connecting Solar Panels in Series

Stringing solar panels in series involves connecting each panel to the next in a line (as illustrated in the left side of the diagram above).

Just like a typical battery you may be familiar with, solar panels have positive and negative terminals. When stringing in series, the wire from the positive terminal of one solar panel is connected to the negative terminal of the next panel and so on.

When stringing panels in series, each panel additional adds to the total voltage (V) of the string but the current (I) in the string remains the same.

One drawback to stringing in series is that a shaded panel can reduce the current through the entire string. Because the current remains the same through the entire string, the current is reduced to that of the panel with the lowest current.

#### Connecting Solar Panels in Parallel

Stringing solar panels in parallel (shown in the right side of the diagram above) is a bit more complicated. Rather than connecting the positive terminal of one panel to the negative terminal of the next, when stringing in parallel, the positive terminals of all the panels on the string are connected to one wire and the negative terminals are all connected to another wire.

When stringing panels in parallel, each additional panel increases the current (amperage) of the circuit, however, the voltage of the circuit remains the same (equivalent to the voltage of each panel). Because of this, a benefit of stringing in series is that if one panel is heavily shaded, the rest of the panels can operate normally and the current of the entire string will not be reduced.

### Information You Need When Determining How to String Solar Panels

There are several important pieces of information about your inverter and your solar panels that you need before you can determine how to string your solar array.

You’ll need to understand the following inverter specifications which can be found in the manufacturer datasheet for the product:

• Maximum DC input voltage (Vinput, max) - the maximum voltage the inverter can receive
• Minimum or “Start” Voltage (Vinput, min) - the voltage level necessary for the inverter to operate
• Maximum Input Current
• How many Maximum Power Point Trackers (MPPTs) does it have?
• As noted above, a function of inverters is to maximize power output as the environmental conditions on the panels vary. They do this through Maximum Power Point Trackers (MPPTs) which identify the current and voltage at which power is maximized. However, for a given MPPT, the conditions on the panels must be relatively consistent or efficiency will be reduced (for instance, differences in shade levels or the orientation of the panels). However, if the inverter has multiple MPPTs then strings of panels with different conditions can be connected to a separate MPPT.

An important thing to understand about these values is that they are based on the module’s performance in what is called Standard Test Conditions (STC). STC includes an irradiance of 1000W per square meter and 25 degrees Celsius (~77 degrees F). These specific lab conditions provide consistency in testing but the real world conditions a PV system experiences may be very different.

As a result, the actual current and voltage of the panels may vary significantly from these values. You’ll need to adjust your calculations based on the expected minimum and maximum temperatures where the panels will be installed to ensure that your string lengths are appropriate for the conditions the PV system will encounter as we’ll discuss below.

## Basic Rules for How to String Solar Panels

### 1. Ensure the Minimum and Maximum Voltage Are Within the Inverter Range

When stringing your solar array, one of the basic considerations is to ensure that the voltage of the strings you are connecting to the inverter is not going to exceed the inverter’s maximum input voltage or fall below its minimum/start voltage. You’ll also need to avoid exceeding its maximum current.

You’ll also need to ensure that the maximum voltage complies with code requirements in the area where you are designing. In the U.S., the National Electrical Code caps the maximum allowable voltage at 600V for most residential systems. In Europe, higher voltages are allowed.

We know that voltage is additive in series strings while current is additive in parallel strings. As such, you might intuitively assume that you can determine the voltage of our proposed PV system design and whether it falls within the recommended range for the inverter by multiplying the voltage of the panels by the number in a series string (as illustrated in the example in the green box below).

 Voltage Maximum and Minimum Calculations based on STC Values (not temperature adjusted): Assumptions: I am using 300W panels with an open circuit voltage (Voc) of 40V. The inverter I plan on using has a maximum voltage (Vmax) of 600 V and a start voltage (Vstart) of 150 V. I can get an initial rough understanding of the maximum number of panels that can be included on a string in series by dividing the inverter’s maximum input voltage by the Voc of the panels: 600 V / 40 V = maximum of 15 panels on a string I can follow the same process, but using the start voltage, to determine the minimum number of panels I can include on a string. 150V / 40 V = minimum of 3.75 panels → therefore minimum of 4 panels on a string BUT, as we discuss below, this doesn’t give the entire picture. You’ll need to adjust based on temperature.

You might also assume that you could determine the current of the system by adding the current of each parallel string (which would be equal to the current of the panels multiplied by the number in the parallel string).

However, as we discussed above, since STC values reflect the modules’ performance under very specific conditions, the actual voltage of the panels in real-world conditions may be quite different. Thus the simplified calculations above only give you an initial rough estimate; you must account for how the voltage of the system will change depending on the temperatures it may experience in the area where it is installed. At colder temperatures, the voltage of the system may be much higher; at higher temperatures, it may be much lower.

To ensure that the temperature-adjusted string voltage is within the input voltage window of the inverter, the following formulas can be used:

Aurora solar design software automatically performs these calculations and alerts you as you are designing if your string lengths are too long or too short given the expected temperatures at the site. (For more information on stringing in Aurora, see this help center article.)

Aurora also performs a variety of other validations to ensure that the system will operate as expected and not violate codes or equipment specifications—this can prevent costly performance issues. (For a detailed overview of these validations see this page in our help center.)

For a real-world example of why it is so important to accurately account for how environmental conditions will impact the voltage of your PV system, read our analysis of an underperforming system in Cathedral City, California. In that case, a solar designer’s failure to account for the presence of shade resulted in the system frequently falling below the inverter’s start voltage and therefore producing significantly less energy than forecasted.

### 2. Ensure Strings Have Similar Conditions—or Connect Strings with Different Conditions to Different MPPT Ports

Once you’ve determined that your strings are acceptable lengths for the inverter specifications, another key consideration is to ensure that the strings to have the same conditions (e.g. same azimuth/orientation, same tilt, same irradiance) if they are connected to the same inverter MPPT.

This is because mismatches in the conditions on the strings will reduce the efficiency and power output of your solar design (for a discussion of why mismatches in shading, orientation, or azimuth result in lost power output see the fourth article in our PV system losses series).

If you are designing for a site where it’s necessary to have panels on different roof faces, or some areas of the array will get more shade than others, you can ensure that the panels with different conditions are separated into their own strings, and then connect those strings to different MPPTs of the inverter (provided your chosen inverter has more than one MPPT). This will allow the inverter to ensure each string operates at the point where it produces the maximum power (its maximum power point).

The above rules will ensure that your stringing configuration will comply with the specifications of your inverter and that the energy production of the system won’t be negatively affected by mismatches in the conditions on the panels.

However, there are additional factors that a solar designer can consider to arrive at the optimal design (that is, the design that maximizes energy production while minimizing cost). These factors include inverter clipping, the use of module-level power electronics (MLPE)—devices which include microinverters and DC optimizers, and design efficiency provided by software tools.

#### Inverter Clipping

Sometimes it may make sense to oversize the solar array that you are connecting to the inverter leading to a theoretical maximum voltage that is slightly higher than the inverter max. This may allow your system to produce more energy (because there are more panels) when it is below its maximum voltage, in exchange for reduced (“clipped”) production during the times when the DC voltage of the array exceeds the inverter’s maximum. If the production gains exceed the production lost to inverter clipping, then you can produce more power without paying for an additional inverter or one with a higher voltage rating.

Of course, this decision should be made with care and a clear understanding of how much production will be clipped compared to how much additional production will be gained at other times. In its system loss diagram, Aurora indicates how much energy will be lost to clipping so that you can make an informed decision about whether this makes sense. For a detailed explanation of inverter clipping and when a system with inverter clipping makes sense, see our blog article on the subject.

#### Module-Level Power Electronics (MLPE)

String inverters are not the only inverter option. Microinverters, which are inverters that are attached to each individual panel (or a couple), allow each panel to operate at its maximum power point regardless of the conditions on other panels. In this arrangement, one need not worry about ensuring panels on the same string have the same conditions. Microinverters can also make it easier to add more panels in the future. We discuss MLPE in more detail in this article.

#### Explore a Few Different Options to Find the Best One

As you can see, there are many considerations when it comes to stringing your panels and finding the inverter and stringing configuration that are best for the customer. You may not arrive at the optimal design the first time around so it can be helpful to evaluate a few different options. In order for this to be efficient, however, you’ll need a process where you can evaluate multiple designs quickly, as Aurora co-founder Christopher Hopper explains in this blog post. This is where solar software can be particularly valuable.

#### Let Solar Software Do the Stringing For You

Finally, new technology developments like Aurora’s autostringing functionality (discussed here) can actually do the stringing for you! It will take into account the considerations discussed here and present you with an ideal stringing configuration.

Understanding the principles of solar panel wiring will enable you to ensure optimal designs for your solar customers. We hope you found this introductory primer helpful!

Green Button data — if you’re not familiar with it, it might sound like something that a Marvel comic book villain would enjoy reviewing. But, in fact, Green Button data is actually a very valuable tool for understanding a home or business owner’s energy usage. In this post, we’ll explore what Green Button data is and what benefits it provides for solar designers and customers.

Green Button data gives utility customers — both residential and commercial — timely access to energy use data in a standardized, computer-friendly format.

#### What is Green Button Data?

Green Button data refers to an option provided by some utilities that enables customers to download detailed data on their electricity usage with just the click of a (green) button from the utility website. Specifically, Green Button data gives utility customers — both residential and commercial — timely access to energy use data in a standardized, computer-friendly format. The Green Button Initiative emerged as a voluntary, industry-led response to a 2011 call-to-action from the White House to make energy data more accessible to consumers.

Beyond showing how much energy a consumer uses, one of the greatest benefits of Green Button data is that it also provides insight into when energy is being used. Historically utility bills have only shown how much energy was used over a monthly period. However with the increased deployment of “smart meters,” which track energy usage at intervals of one hour or less, much more granular energy use data is becoming available. If a customer has a smart meter, Green Button data will allow a customer to see exactly how much energy they use at specific intervals.

The measurement interval available to customers depends on what their utility offers, but many utilities — especially in California and Texas, where utilities are required to provide customers with their energy usage data — provide this information in 15-minute increments. Currently, more than half of American households have smart meters and they are increasingly being deployed by utilities around the country as part of utility efforts to modernize the electric grid. Smart meters and Green Button data go hand in hand as methods to give customers’ greater insight into and control over their energy use. The Green Button program is helping to make the improved data from smart meters more easily accessible.

Beyond showing how much energy a consumer uses, one of the greatest benefits of Green Button data is that it also provides insight into when energy is being used.

#### Why Is Green Button Data Important?

Green Button data offers numerous benefits to energy consumers and for solar professionals looking to design and sell high quality solar installations. Green Button data helps consumers better understand when they are consuming energy, and save on their utility bills. For instance, for residential customers in areas where Time of Use (TOU) rates are standard (like California), Green Button interval data can show how changing the timing of certain energy-intensive activities can result in reduced energy bills.

Green Button data is particularly useful for customers who are considering solar, because it makes evaluating projects and savings faster and more accurate.

Green Button data offers additional value for commercial customers. Beyond the insights it provides with regard to Time of Use rates (which are more common for commercial customers), Green Button data can also help commercial customers better understand demand charges, which are fees a utility charges based on the maximum amount of power a commercial customer consumes over a given time period.

An example of a customer's hourly energy usage based on Green Button Data uploaded in Aurora.

#### Green Button Data and Solar: A Perfect Combination

Green Button data is particularly useful for customers who are considering solar, because it makes evaluating projects and savings faster and more accurate. Having a clear picture of a household’s energy consumption is critical to determining the appropriate size of a solar array. Furthermore, precise data on energy consumption at different times throughout the day is important in enabling accurate evaluation of the financial returns of the solar design.

For instance, if the customer is billed under Time of Use rates, in order to understand how much a solar installation will reduce their utility bill, it is essential to understand how much energy they consume during peak demand times when energy is more expensive, and how those usage patterns intersect with the amount of energy their solar array is likely to be producing at different times. A customer’s savings will be greater if the energy produced by their solar installation coincides with and can offset much of their electricity consumption during hours when electricity is most expensive (typically in the afternoon). Furthermore, this consideration might influence the ideal location or orientation of a solar design (such as siting the design where it will get more afternoon light, and thus offset energy when electricity prices are higher, rather than where it would produce the most energy overall).

For commercial customers, whose utility bills include demand charges, the benefits of using Green Button data in the solar design process are a little more nuanced, so we will cover them in a later post.

With a customer’s Green Button data, you can save time by automatically importing the exact details of the customer’s energy consumption and Aurora will use that to model the customer’s electricity usage throughout the day and throughout the year (their load profile). Combined with Aurora’s simulations of the solar design’s energy production (the industry’s most accurate), you and your solar customer can be confident in the expected financial return on the installation.

#### Key Takeaways:

• The Green Button Initiative is a program through which participating utilities provide customers with detailed data on their energy usage, in a standardized, machine-readable format.
• Green Button data gives utility customers greater insight into the amount and timing of their energy consumption, helping them to understand how they can save energy and reduce their utility bills.
• Green Button data is particularly useful in helping potential solar customers accurately evaluate the financial return on a solar installation.
• Aurora’s software can automatically import and interpret Green Button data enabling faster and more accurate development of detailed solar sales proposals.

~~~
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## Solar Industry Growth and Affordability

Solar is playing an increasingly important role in the transition to a world powered by renewable energy. Over the past decade, the number of solar installations has grown at an accelerating rate and with increasing affordability. In the first quarter of 2016, over 29 GW of solar was installed in the United States.

Figure 1 illustrates the correlation of the increasing number of installations with the decreasing costs. It shows that the price of a solar installation is now less than a third of what it was in 2009, while annual installations have grown more than tenfold during the same period of time.

Figure 1. Annual solar installations in the US are accelerating as price decreases. Source: SEIA

SEIA states that the solar industry is a powerful engine for economic growth. The US solar industry currently employs over 200,000 people, twice as many as in 2010 and now employs more people than the coal, or the oil and gas industries. As installed capacity continues to increase, SEIA predicts that the solar workforce will expand to 420,000 by 2020.

Figure 3. The solar industry now employs more workers than coal mining and petroleum extraction. Source: SEIA

## Solar Energy, Power, and Irradiance

Solar panels convert the energy of photons, or light particles, from the sun into electricity. Photovoltaic devices, such as solar panels, permit the incoming photons to transfer their energy to electrons. These energized electrons begin to flow, creating an electric current. We use the terms irradiance or insolation to refer to the power density of sunlight on a surface.

Figure 4. Basic schematic of a PV array. Source: clean-energy-ideas.com

We typically measure energy in kilowatt-hours (kWh), and power (the rate at which energy is produced) in kilowatts (kW).

$$\text{Energy} = \text{Power }\cdot \text{Time} = 1\mathrm{kW} \cdot 1 \mathrm{hour} = 1\mathrm{kWh}$$

In solar, we usually define the size of a solar installation in terms of its power (in kW).

Irradiance is typically reported in units of kilowatt-hours per meter squared per day (kWh/m2-d). The amount of irradiance hitting the surface of the earth is often quoted in terms of the number of hours of “full-sun” of solar energy. A "full-sun" is defined as 1 kW/m2.

Quantity Units Definition
Power kW Rate of energy production/output
Energy kWh Capacity to do work
Irradiance kWh/m2-d Hours of full-sun for a square meter each day
Table 1. Important quantities used for solar energy

## Solar Resource of a Rooftop

We can estimate the solar potential of a rooftop using its area and the local irradiance. NREL, the National Renewable Energy Laboratory, publishes irradiance data in its report Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors.

Figure 5. Annual irradiance value for a 150m2 roof plane in Palo Alto. Source: Aurora Solar

It is fairly straightforward to calculate rooftop solar potential of a rooftop using this data. For example, a south-facing roof plane of a home in Palo Alto, CA (Figure 5) receives an average irradiance of approximately 1,900 kWh/m2/year. Dividing the annual irradiance value by the number of days in a year yields the average daily irradiance.

$$\text{Average Daily Irradiance} = \frac{\text{Annual Irradiance}}{\text{days/year}} = \frac{1900 \mathrm{kWh/m^2year}}{365 \mathrm{days/year}} = \mathrm{5.2 \mathrm{ kWh/m^2day}}$$

To calculate the amount of solar energy available on a roof face, multiply its area by the average irradiance value.

$$\text{Rooftop Energy}\mathrm{[\frac{kWh}{day}]} = \text{Irradiance}\mathrm{[\frac{kWh}{m^2 \cdot day}]} \times \text{Area}\mathrm{[m^2]}$$

If the rooftop has an area of approximately 150m2, the solar energy available on the rooftop is as follows.

$$\text{Rooftop Energy} = 5.2 \frac{\mathrm{kWh}}{\mathrm{m^2}\cdot\mathrm{day}} \times 150\mathrm{m^2} = 780 \mathrm{\frac{kWh}{day}}$$

Besides the solar irradiance, Figure 5 also displays information on three additional quantities related to the solar resource: Solar Access, TOF, and TSRF:

Solar Access: This is the ratio of the actual solar energy available — taking into account shading cast by objects in the environment — to the solar energy that would be available in the absence of shading. You can learn more about the effects of shading on PV systems here.

$$\text{Solar Access} = \frac{\text{Energy with Shade}}{\text{Energy without shade}}$$

TOF (Tilt and Orientation Factor): This is the ratio of the amount of solar energy a location receives to the amount it would receive if the orientation of the roof were optimal.
$$\text{TOF} = \frac{\text{Energy with actual tilt and orientation}}{\text{Energy with optimal tilt and orientation}}$$

TSRF (Total Solar Resource Factor): This is the percentage of the available solar resource that a location receives as compared to what it would receive with optimal orientation and without shading. TSRF is equivalent to the Solar Access multiplied by the Tilt and Orientation Factor.
$$\text{TSRF} = \text{Solar Access}\times \text{TOF}$$

### About Solar PV Education 101

The Beginner's Guide to Solar Energy is part of Solar PV Education 101, a six-article series that serves as an introductory primer on the fundamentals of solar PV for beginners.

## Sizing a PV System from an Electricity Bill

An electricity bill typically reveals information about a residential or commercial customer’s total monthly energy consumption (as we discussed in the previous article in this series, Reading Your Electricity Bill: A Beginner’s Guide). From this value alone, it is possible to approximate the required size of a PV system that offsets monthly energy usage.

Take a hypothetical monthly energy consumption of 500 kilowatt-hours, which is on the lower end for a household in California. Assuming there are 30 days in a month, an average daily energy use value can be reached by dividing the monthly use by 30.

$$\text{Daily Energy Use} = \frac{\text{Monthly Energy Use}}{\text{Days in Month}} = \frac{500 \mathrm{kWh/mo}}{30 \mathrm{days/mo}} = 16.7 kWh/day$$

Next, insolation values are needed. As mentioned in The Beginner’s Guide to Solar Energy, insolation values are reported in kWh/m2-day. Since a “full-sun’s” worth of incoming solar energy is approximated as 1 kW/m2, insolation values reported in kWh/m2-day approximate the hours of full-sun equivalent that a location receives over the course of a day.

Figure1. Visualization of how total solar insolation received over the course of a day (left) can be represented by number of full-sun hours (right). Source: pveducation.org

For a Palo Alto home, the average daily irradiance value is 5.2 kWh/m2-day. By dividing the daily energy usage by hours a day of full sun, the power output required by the PV system is calculated.

$$\text{Power Output} = \frac{\text{Daily Energy Use}}{\text{Daily hours of full sun}} = \frac{16.7 \mathrm{kWh/day}}{5.2 \mathrm{hours/day}} = 3.21 \mathrm{kW}$$

Figure 2. The Palo Alto home used for this PV system sizing exercise. Source: Aurora Solar

This would be the size of the PV system required, if our system was 100% efficient. However, that is not the case because all PV systems have a corresponding derating factor that takes into account the inefficiencies of the overall system, such as soiling of the panels and imperfect electrical connections.

According to the National Renewable Energy Laboratory’s PVWatts calculator, a typical derate factor is 0.84. For the sake of this calculation, we assume the derate factor be 80%, or 0.8. In order to determine the size of the PV system, divide the required power output by the derate factor.

$$\text{PV System Size} = \frac{\text{Power Output}}{\text{Derate Factor}} = \frac{3.21 \mathrm{kW}}{0.8} = 4.01\mathrm{kW}$$

From this analysis, the approximate size of a PV system required to completely offset the average monthly energy usage of a 500 kWh/month home in Palo Alto would be about 4 kW.

## Comparing the PV Size Estimation to a Simulated Result

Since this is a rough estimate, how does it compare against an actual, comprehensive design for a home with the same characteristics?

Using the same conditions as above, a PV system design software found that the required system size to be 4 kW, which is almost identical to the answer from the estimation conducted above.

Although the answers are very close, it’s important to note that this may not always be the case. For instance, when there is shading on the panels, a significant reduction in power output can occur. Although a shading term is included when calculating a derate factor, it can fail to accurately capture the effect that shading has on a PV system’s power output. Therefore, expect the results to be less close when modeling a location with shading.

### About Solar PV Education 101

How to Size a PV System from an Electricity Bill is part of Solar PV Education 101, a six-article series that serves as an introductory primer on the fundamentals of solar PV for beginners.

## Effects of Shade on PV Output

Since PV systems generate electricity based on the amount of sunlight they receive, it makes sense that when a shadow is cast on a panel, for example by a nearby tree, its power output decreases. However, the decrease in power could be a lot worse than it initially seems.

Figure 1. Solar panels in partial shade. Source: lowcarbonlivingblog.wordpress.com

Intuition suggests that power output of the panel will be reduced proportionally to the area that is shaded. However, this is not the case. In his book Renewable Energy and Efficient Electric Power Systems Stanford University’s Gil Masters demonstrates how shading just one out of 36 cells in a small solar module can reduce power output by over 75%.

## Waterflow Analogy

To conceptualize why shading results in such severe losses, it is helpful to use the analogy of water flowing in pipes. The flow rate of water through the pipe is constant, much like the current through a cell string is constant for a given irradiance level.

Figure 2. Analogy of a water pipe to a string of solar cells.

Shading a solar cell is similar to introducing a clog in a pipe of water. The clog in the pipe restricts the flow of water through the entire pipe. Similarly, when a solar cell is shaded, the current through the entire string is reduced.

This is significant because every cell in the cell string has to operate at the current set by the shaded cell. This prevents the unshaded cells from operating at maximum power. Therefore, only a small amount of shading can have a dramatic effect on the power output of a solar panel.

Similar principles apply to PV modules connected together. The current flowing through an entire string of modules can be heavily reduced if even just a single module is shaded, leading to potentially significant loss of power output.

## Approaches to Reduce Shading Losses

Fortunately, there are a number of different approaches that can be applied in PV system design to reduce shading losses. These include the use of different stringing arrangements, bypass diodes, and module level power electronics (MLPEs).

### Stringing Arrangements

Modules connected in series form strings, and strings can be connected in parallel to an inverter. The current through all the modules of a string has to be the same, and the voltage of parallel strings has to be the same. As we saw in the last section, a shaded module in a string can bring down the power output of the string significantly. However, a shaded module in one string does not reduce the power output of a parallel string. Therefore, by grouping shaded modules into separate strings, the overall power output of the array can be maximized.

For example, in a commercial system with parapet walls, it can be beneficial to group modules that receive shade from the parapets into strings, and keep modules that do not receive shade from the parapets in separate, parallel strings. This way the unshaded strings can maintain a higher current and power output.

Figure 4. PV arrays with modules connected in series (left) and in parallel (right).

### Bypass Diodes

Bypass diodes are devices within a module that allow the current to “skip over” shaded regions of the module. By utilizing bypass diodes, the higher current of the unshaded cell strings can flow around the shaded cell string. However, this comes at the expense of losing the output of the cells that are skipped over.
Although it would be theoretically ideal to have a bypass diode for each solar cell, for cost reasons a typical solar module will have three bypass diodes, effectively grouping the cells into three series cell strings (Figure 5). For instance, a 60-cell module will typically have one bypass diode for every 20 cells.

Figure 5. PV module containing three cell strings in series, each with a parallel bypass diode.

### Module Level Power Electronics (MLPEs)

MLPEs are devices that are attached to individual modules in order to increase performance under shaded conditions (though there are other benefits, such as mismatch mitigation and module-level monitoring). This is done by performing maximum power point tracking at the module level. MLPEs include DC optimizers and microinverters.

DC Optimizers:

A DC optimizer adjusts its output voltage and current to maintain maximum power without compromising the performance of other modules.

For instance, when a shaded module produces electricity with a lower current, the DC optimizer will boost the current at its output to match the current flowing through the unshaded modules; to compensate, the optimizer reduces its output voltage by the same amount it boosts the current. This allows the shaded module to produce the same amount of electrical power without impeding the output of other modules. A system utilizing DC optimizers still needs an inverter to convert electricity from DC to AC.

Microinverters:

As opposed to having a single inverter servicing all of the panels, each panel can have a small inverter attached to it to convert its output from direct current (DC) to alternating current (AC). Since each microinverter has an MPPT, and their outputs are connected in parallel, each panel will operate at its maximum power point, without impacting other panels.

Figure 6. Simplified schematic of a PV system utilizing microinverters (top) and a PV system utilizing DC optimizers (bottom).

## Effects of MLPEs on PV System Performance

Using Aurora’s simulation engine, we compared the performance of three different PV systems subject to significant shading. As shown in Figure 7, we placed a 3.12 kW system near the edge of a roof, which has tall trees next to it. Note that while this design effectively showcases the performance difference of these system topologies in shaded conditions, it is not an optimal—or even a practical — design. Our findings are summarized in Table 1.

Figure 7. The system analyzed for this case study featured a 3.12 kW system that is partially shaded by trees.

Table 1. Results from performance simulations of PV system on a Palo Alto home utilizing different MLPE components. The difference between the two MLPE outputs is attributed to the differences in their inverters' efficiencies. Source: Aurora Solar.

System Topology Annual Yield Improvement with MLPEs
String Inverter 2,585 kWh/year N/A
Microinverters 3,033 kWh/year +17.3%
DC Optimizers 3,035 kWh/year +17.3%

Our results show that using MLPEs under these conditions increases system output by 17.3% annually, showing the benefit of using these components for shade mitigation. Additionally, the effective yield of a system using a microinverter or a DC optimizer is approximately the same, although there could be small differences (on the order of 1%) in some cases due to differences in efficiency curves.

For the same reason that they can mitigate shade losses by decoupling module output, MLPEs can eliminate module-to-module mismatch losses. These losses are typically caused by manufacturing variations that lead to slight differences in the electrical characteristics of two modules of the same type. Since MLPEs allow the modules to operate independently from one another, these variations will not impact the system’s overall performance.

### About Solar PV Education 101

Article 5: Shade Losses for PV Systems, and Techniques to Mitigate Them is part of Solar PV Education 101, a six-article series that serves as an introductory primer on the fundamentals of solar PV for beginners.

## Costs Associated with a PV System

In order to determine financial returns, it is important to have a solid understanding of the basic economics that dictate PV system costs. There are two general categories of PV systems costs: capital costs and operation and management (O&M) costs.

### Capital Costs

Capital costs refer to the fixed, one-time costs of designing and installing the system. Capital costs are categorized into hard costs and soft costs.

Hard costs are the costs of the equipment, including modules, inverters, and BOS components, as well as installation-related labor.

Soft costs include intangible costs such as permitting, taxes, customer acquisition costs, etc.

Figure 1. Cost breakdown of PV systems. Source: B. Fiedman, et al, "Benchmarking Non-hardware BoS Costs for US PV Systems, Using a Bottoms-Up Approach and Installer Survey," National Renewable Energy Laboratory, Second Edition, December 2013.

Figure 1 illustrates the relationship between soft and hard costs, and breaks down hard costs into its components. According to SEIA, while hard costs have come down dramatically over the last decade, soft costs have remained largely constant.

### Operation and Management Costs

O&M costs refer to costs that are associated with running and maintaining the system. These can include fuel, repairs, and operation personnel. PV systems generally have low O&M costs.

## Incentives and Policies that Benefit Solar Energy

The high capital costs are one of the biggest factors that discourage people from going solar. To combat this, there are a number of incentives and policies in place to make PV systems financially competitive.

### Cost-Based Incentives

Cost based incentives, such as the Solar Investment Tax Credit (ITC), allow those who invest in a solar system to apply a tax credit towards their income tax. The incentive is determined by the cost of the system, and is independent of its performance.

### Performance-Based Incentives

Performance based incentives (PBIs) encourage PV system owners to install and maintain efficient systems through payments that are based on the monthly energy production of the system.

### Net Energy Metering

In addition to incentives, many states, such as California, implement a net energy metering (NEM) policy that allows consumers who generate excess electricity to be reimbursed at the then-prevailing rate of electricity. For instance, if a residential PV system produces an excess of 100 kWh over the course of the month, the owner will be reimbursed for 100 kWh at the market rate of electricity for that time period. The owner is then free to use that reimbursement credit towards electricity they consume from the grid when solar is not meeting their current energy load. Therefore, households with solar PV and NEM are able to significantly reduce their electricity bill.

Figure 2. Visualized relationship between PV energy production and household electricity use for an average home in New South Wales, Australia. Source: solarchoice.net.au

Figure 2 shows the relationship between PV electricity production and electricity consumption during the day. Note that while the PV system can generate more than enough electricity during the daytime, it can fail to deliver electricity during peak consumption hours.

## Basic Financial Calculation for a Residential PV System

In return for a large upfront investment in a solar installation, homeowners that go solar benefit from a reduced monthly electricity bill. Thus, for NEM regimes the benefit of solar comes in the form of avoided costs.

For instance, assume that upon installing a rooftop PV system, a home electricity bill is reduced by $1,500 per year and the cost of the hypothetical PV system is$10,000 after incentives. In order to calculate the simple payback period, which is the approximate time for a PV system to pay for itself, we divide the cost of the PV system by the savings.

$$\text{Simple Payback Period} = \frac{\text{System Cost}}{\text{Annual Savings}} = \frac{10,000}{1,500\mathrm{/year}} = 6.7\mathrm{years}$$

Thus, the payback period for a system that costs $10,000 and reduces the electricity bill by$1,500 per year is 6.7 years.

However, a PV system can last much longer than the duration of its payback period. A typical rooftop PV system has a lifetime of about 25 years. This means that for the last 18 years of its life, after it has paid itself off, the hypothetical PV system described above will generate revenue in the form of additional savings. To calculate this revenue, we multiply the annual savings by the remaining lifetime of the system, after it has paid itself off.

$$\text{Net Revenue} = \text{Annual Savings} \times \text{Years left in lifetime after system is paid of}$$ $$\text{Net Revenue} = 1,500\mathrm{/year} \times 18.3\mathrm{year} = 27,450$$

Based on this simple analysis, the system will generate approximately $27,450 in savings over its lifetime. It is important to note that this is an approximation, and does not take into account factors such as maintenance costs, changes in electricity price and usage, as well as system degradation over time. The figure below shows another financial analysis for a hypothetical residential PV system. In both graphs, the y-axis is the dollar amount and the x-axis is the year. Figure 3. The cumulative (top) and annual (bottom) cash flows of a hypothetical PV system. Source: Aurora Solar The top graph, which shows the cumulative cash flow of the project over time, and indicates that the project has a payback period of approximately four years. Additionally, the dollar amount in the 25th year, which is about$25,000, is the cumulative net revenue that the system generated. The bottom graph is the annual cash flow of the project. The first year is characterized by a large negative cash flow, due to the large upfront cost required to install the system, but after that there is positive annual cash flow with the exception to this is in the 14th year, which is when the inverters are being replaced.

### About Solar PV Education 101

The Basic Principles that Guide PV System Costs is part of Solar PV Education 101, a six-article series that serves as an introductory primer on the fundamentals of solar PV for beginners.