Solar energy is the radiant electromagnetic energy - or light - received from the Sun by the Earth. While the solar radiation received by the Earth's atmosphere is relatively constant, the amount of sunlight hitting the Earth's surface varies widely due to many factors including:
- atmospheric effects, including absorption and scattering
- local variations in the atmosphere, such as water vapor, clouds, and pollution;
- latitude of the location
- the season of the year and the time of day
The above effects have several impacts on the amount and quality of the solar energy that we actually receive, including variation in the amount of power, the specific wavelengths of light and the angle at which sunlight strikes the Earth's surface (incident angle of sunlight). In addition, the variability of the solar radiation at a particular location will be affected by the above considerations. For instance, desert regions tend to have lower variations due to local atmospheric phenomena such as clouds. Equatorial regions have low variability between seasons.
To quantify solar resource, we measure the amount of power received from the Sun over a given area of earth. This value is called irradiance and is measured in kilowatts per square meter. "Peak sun" is used to refer to the time of day where a location typically receives 1 kW/m2 of power, which is equivalent to 1,000 W/m2. Peak sun conditions are used as a standard to rate the amount of electrical power a photovoltaic module will produce. A 200-Watt photovoltaic module will therefore produce 200 Watts of electrical power when receiving 1,000 W/m2 of solar power.
Cumulative irradiance is known as insolation and is the amount of energy received from the Sun over a given area of earth for a given period of time and is measured in kilowatt-hours per square meter. Insolation is often measured as "daily insolation" or "annual insolation."
If a certain area receives 1 kW/m2 for 12 hours and 0 kW/m2 for 12 hours, then over a 24 hour period (daily insolation) would be:
1 kW/m2 x 12h + 0 kW/m2 x 12h = 12 kWh/m2