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The Beginner’s Guide to Solar Energy

Christian BrownChristian Brown

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.

Annual solar installations in the US are accelerating as price decreasesFigure 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.

Graph of solar workers vs. gas and coal from 2010 to 2016Figure 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.

Basic schematic of a PV arrayFigure 4. Basic schematic of a PV array. Source:

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.

Annual irradiance values on a roof plane in Palo AltoFigure 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.

Article 1: The Beginner's Guide to Solar Energy
Article 2: How a Photovoltaic System Produces Electricity
Article 3: Reading Your Electricity Bill: A Beginner’s Guide
Article 4: How to Size a PV System from an Electricity Bill
Article 5: Shade Losses for PV Systems, and Techniques to Mitigate Them
Article 6: The Basic Principles that Guide PV System Costs