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The single most important thing to determine when buying a PV power system is how much electricity it will generate for your home.
This requires calculating of the average amount of the sun's energy that reaches the earth in your area. So, let's start with the basic facts. The yearly average density of Sun's irradiance at the top Earth's atmosphere on an imaginary surface perpendicular to the sunlight rays is approximately 1366 watt per square meter (see solar energy diagram below). This value is called solar constant (read more). Note that in reality, it is not really a constant: it slightly varies throughout the year and also through 11-year cycles.
You can see from this diagram that the net amount of power intercepted by the Earth at any give moment equals to solar constant times Earth's cross-sectional area Ac=πd2/4, where d is the Earth diameter, π=3.14. We know that the total surface area of a sphere is πd2. Therefore if we take the amount of sunlight power that reaches our atmosphere and average it over entire Earth's surface, we'll get of solar constant, which is about 341 W/sq.m. About 29% of this amount is reflected back to space, 23% is absorbed in the atmosphere, and 48% reaches the surface.

Solar energy diagram

Thus, an average amount of sunlight received at the Earth surface per square meter is 3410.48=163 W/sq.m, or 15 watt per square foot. Note that this is the value averaged over the entire surface of the planet. Of course, the incoming energy is not distributed evenly. The closer to the equator, the more sunlight is available throughout the year. The solar power density at the equator on a bright day at noon is about 1000 watts per square meter. This value is called the "standard sun". It is used in the industry for rating efficiency and peak power output of PV panels. Outside of the equator you can probably get this wattage on a surface directly facing the sun (i.e. tilted toward sun according to your latitude) briefly at a bright noon. Remember however, this is just a peak power. During a day, the sun's radiation varies from a maximum value to zero as the sun moves across the sky. That is why a more important characteristic of solar irradiance is the net amount of the sunlight delivered during an entire day.


The net amount of the sunlight received during a day varies significantly with geographical locations and the weather patterns. To calculate the average amount of electricity a residential PV system can generate, you need to know the characteristic called insolation (INcoming SOLar radiATION). Insolation levels represent daily average solar energy. They are usually expressed in kilowatt-hours per square meter per day (kW-hr/m2/day), or as an amount of equivalent hours of "standard sun". Numerically both values are the same. NASA maintains an extensive database on insolation for most locations in the world. NREL laboratory has the maps for the U.S. For more details see practical data and facts about solar energy. Note that in today's non-concentrated PV systems for homes only less than 21% of the sunlight can be practically converted to electricity because of a limited efficiency of any PV power system.

U.S. solar radiation resource maps and data manual
NASA worldwide insolation data