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Georgia Tech students install "light walls" on their energy-efficient home displayed at the National Mall's 2007 "Solar Village."

Photo courtesy of EERE

Contrary to what most people think, the size of a solar-power installation has nothing to do with the size of the house it's powering.

Instead, it's all about two things: insolation, which we just discussed, and how much power you need. If you live in Albuquerque and you don't use much power, you need a smaller system. If you live in Chicago and run a circa-1980 electric furnace all day every day, you're looking at a lot more panels on your roof.

To get a very rough estimate of how big a system you need, look at your electric bill and figure out how many kWh you use per day. The average home in the United States uses about 900 kWh every month, or about 30 kWh per day [source: DOE]. Multiply that by 0.25 [source: GE]. We come up with 7.5, so we need a 7.5 kW system.

A typical solar panel produces a maximum of 120 watts, or 0.12 kW, in a day [source: Richards]. For a 7.5-kW setup, then, you'd need about 62 panels. A single panel might measure about 56 by 25 inches (142 by 64 centimeters), so a 62-panel solar setup would measure roughly 700 square feet (65 square meters).

There's more that goes into this calculation, though. You have to figure in insolation and how many hours of peak sunlight you get per day, and you'd also make adjustments if you're using a battery-storage system with the panels. So it's best to consult a pro.

But assuming this is roughly the system you'd need to power your home, how much of a financial investment are we looking at for a 7.5-kW solar setup?

Up next, perhaps the greatest consideration of all: the expense.

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