A popular story in The New York Times this week documents the rising trend of "passive houses"-homes that are built without furnaces and instead are constructed to retain heat generated by the sun, appliances, and the occupants themselves. They're slightly more costly to build, but the savings-both financial and environmental-are pretty staggering.It's an exciting trend, to be sure, but not everyone is in the market to build a new house. So what can you do to your existing abode? One answer is to add solar panels. Historically, this has been an intimidating prospect, mainly because of the high cost associated with installation. But a new site, RoofRay, let's you estimate how much power you could potentially generate from the sun, and how much it could save you each year. You input your address, then use RoofRay's tools to map out the shape of your roof. In addition to estimating annual savings, the site lets you compare various different installation options and chart your return on investment. At worst it's a fun awareness tool-I, for example, feel great about my 8.97 watts-per-square-foot potential-but at its best, it's a way to understand the options available to you when purchasing a new house, or renovating your current one.