Agreed PVWatts is more accurate, and it's what a solar installer is actually likely to use when doing a roof assessment. But to use PVWatts you need to input things like the tilt and orientation of your roof, the size of the system you want to build, the cost of power from your provider, inverter efficiency, etc. This asks for your address and your monthly electricity bill to give you a rough estimate, both of which homeowners are much more likely to know.