The sun shines on everyone — but not in equal measure. That reality has long slowed the spread of solar power. Depending on where you live in the country — or even where you live in your city — the same array of photovoltaic solar panels can produce enough electricity to power your house with watts to spare, or barely cut a nickel from your utility bill. It all comes down to the precise amount of sunlight that hits your roof. But while we all know that San Antonio gets more sunny days than Seattle, what about one part of San Antonio compared to another? One block of downtown Seattle compared to the next block? "Without that knowledge, renewables can be a bit of a crap shoot,' says Kenneth Westrick, the CEO of the renewable mapping company 3Tier. (See TIME's Top Ten Green Ideas of 2008)

All of that could be changing. The engineering company CH2M Hill is now joining hands with the U.S. Department of Energy to provide Internet solar maps of 25 American cities, using Google Earth technology to chart the precise solar potential of neighborhoods, literally rooftop by rooftop. The company has just finished mapping all of San Francisco, allowing residents to enter their address and take the solar measure of their own home. "People in San Francisco think we don't have any solar potential,' says Gavin Newsom, the city's deep-green mayor. "But the map shows we have a lot more sun than you'd believe."