Do anyone know of an online site where you can calculate usable power from a given sized solar cell?

If you have an out building that needs basic electric power and it is too far from the main building as to require stringing more than 80ft of extension cord (not desired) how do you calculate how large of a solar sell, or how many you would need? If the use is an occasional use of circular saw, laptop (3 hours daily tops), charging batteries for cordless circular saw, and a couple of drills, small fan in hot weather and small heater for three hours in winter, how do you calculate how large of a watt solar panel you would need, or how much usable power you would get from 2 500w panels, etc.? Is there a site out there where one can use to determine the gain of different panels?