The Solar Resource
Solar electricity generation represents a clean alternative to electricity from fossil fuels, with no air and water pollution, no global warming pollution, no risks of electricity price spikes, and no threats to our public health.
Solar energy can also heat water, cool and heat homes, and provide free, natural lighting. And once a system is in place to convert the solar resource into useful energy, the fuel is free.
The solar resource is enormous
Just 18 days of sunshine on Earth contains the same amount of energy as is stored in all of the planet's reserves of coal, oil, and natural gas.
Outside the atmosphere, the sun's energy contains about 1,300 watts per square meter. Once it reaches the atmosphere, about one-third of this light is reflected back into space, while the rest continues toward Earth’s surface.
Averaged over the entire surface of the planet, a square meter collects 4.2 kilowatt-hours of energy every day, or the approximate energy equivalent of nearly a barrel of oil per year.
Deserts, with very dry air and little cloud cover, receive the most sun—more than 6 kilowatt-hours per day per square meter on average over the course of the year.
Converting solar energy into electricity
Rooftop PV panels make solar power viable in virtually every part of the United States. In a sunny location such as Los Angeles or Phoenix, a five-kilowatt residential system produces an average of 7,000 to 8,000 kilowatt-hours per year, roughly equivalent to the electricity usage of a typical U.S. household.
Solar achieves similar results in many other parts of the country as well. For example, in some northern locations—such as Portland, Maine—that same system generates 85 percent of what it would in Los Angeles on average, and 95 percent of what it would in Miami. (And the system in Maine would actually generate 6 percent more electricity than in Houston ).
As of mid-2015, almost 800,000 PV systems had been installed on rooftops across the United States . And the potential is far greater, with some 35 million residential and commercial rooftops suitable candidates for PV .
Like rooftop panels, large-scale PV projects use photovoltaic panels to convert sunlight into electricity. These projects often have outputs in the range of hundreds of megawatts, which can involve millions of solar panels installed over a large area of land. In addition to scale, outputs can be increased with specialized tracking mechanisms that allow panels to follow the sun and collect light at an optimal angle, greatly increasing the system’s efficiency. Concentrated solar power (CSP), instead of using the sun’s light, takes advantage of the sun’s heat to produce a heated liquid and steam that drives a turbine. While CSP projects have been installed in various parts of the country, the CSP resource is largely concentrated in the desert regions of the U.S. West and Southwest, where the solar resource is more consistent and concentrated.
In 2014 alone, for example, three large projects went online in the desert environments of California—Mojave Solar One, Ivanpah Solar Electric Generating System, and Genesis Solar—adding a total of 892 MW of solar capacity to the electricity grid, enough power to serve nearly 220,000 average American homes .Given the abundance of sunshine across the country, solar has the potential to supply a rapidly growing amount of electricity that is environmentally and economically attractive, nationwide.
 National Renewable Energy Laboratory (NREL). 2014a. PVWatts® calculator. Golden, CO.
 Solar Energy Industries Association (SEIA). 2014d. Solar Industry Data.
 Environment America. 2014. Star Power, The Growing Role of Solar Energy in America.
 U.S. Department of Energy (DOE). 2014. 2014: The Year of Concentrating Solar Power.