Insolation Rating

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Insolation Rating

Employees of a solar company install panels on a residential rooftop in Santa Monica, Calif.

David McNew/Getty Images

Sunlight is obviously key when it comes to solar power, and not all regions are created equal in this regard. It's important to know how much sunlight reaches ground level in the area where your potential solar house is located.

What we're talking about here is insolation -- a measure of how much solar radiation hits the ground in any given area in a specified time period. It's typically measured in kWh/m2/day, and it tells you how much sunlight will be available for your solar panels to turn into electricity. The higher the insolation value of your location, the more electricity each of your panels will be able to generate. A high insolation value means you can get more power out of fewer panels. A low insolation value means you could end up spending more to achieve the same power output (more on expenses later).

If you live in a place like Phoenix, Ariz., or Albuquerque, N.M., you're golden. They've got super high insolation ratings, 6.58 and 6.77, respectively [source: CleanBeta]. This means in Albuquerque, 6.77 kWh worth of sunlight falls on each square meter of Earth in the course of one day (on average). In Portland, Maine, on the other hand, one square meter of land receives 4.51 kWh worth of sunlight in a day [source: CleanBeta]. And Chicago sees just 3.14 kWh/m2/day [source: CleanBeta].

Does that mean you have to build your solar-powered home in the Southwest instead of the Midwest? Not at all. It just means that in Chicago, a solar setup is going to be less efficient than a similar setup in Albuquerque. You're probably going to need more panels to achieve the same power output.

That brings us to No. 2 on the list: How many panels do you need?

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