Area of Panels
This is the total area the panels will take on your roof. The foot-by-foot calculation is just to show roughly how large the area would be if it were exactly square.

You might find it interesting to increase the number of panels until the area is equal to the area of your entire roof. You may find that if you had a roof that entirely faced south toward the sun, and was entirely covered with solar panels, that this would be enough to cover all of your average electricity needs. Unfortunately, the subsidy is probably not enough for this in Austin where we use air conditioners much of the year.

Another interesting calculation is to increase the number of panels to about 30 billion (30000000000) panels. This would take roughly 10,000 square miles of area, a 100 mile by 100 mile square in west Texas, and on average enough to power the electic needs of the whole country.