As an additional method of calculating energy use and required PV paneling, Coldham and Hartman Architects came up with a very rough rule of thumb.

For this method, you take your annual total energy use in kBtu per year (there are energy modeling programs that can help you with this). You then divide this by 3.412 to get your annual kWh usage (or you could pull that figure off your accumulated bills for all your energy sources).

Divide that figure by 1.1 to convert it to PV Watts (which are slightly different) and divide that figure by 10 to give you a very rough number of square feet of paneling you'll need.

It's a very vague estimate but it's quick and easy!