Here is a link to the solar calcualtor.

I wondered about the accuracy of the solar calculator listed above. Bert, in the Houston area, has a PV installation of 3.8 k-Watt. He has monitored the kWatt-hrs collected over the 26 months, since installation. For 2008, his data indicates the harvested 4520 kWatt-Hr. The link above predicts an annual harvesting of 4635 kWatt-hrs. Bert's data, in this case is 3% off. He has a slight amount of tree shading.