If you just want to know the maximum number of panels that can fit on your rooftop, you can fit approximately 10 watts of solar per square foot on a flat unobstructed rooftop. For instance, 100,000 square feet x 10 watts per square foot = 1,000,000 watts or 1MW. This is equivalent to (2,000) 500-watt solar panels.
Pitched rooftops can fit approximately 14-16 watts per square foot. For instance, 100,000 square feet x 15 watts per square foot = 1,500,000 watts or 1.5MW. This is equivalent to (3,000) 500-watt solar panels.