Just total up the number of watts you need. Don't forget the surge watt, the generator has to be able to hand the surge watt requirement of the appliance. For example, a refrigerator might only need 700 watts to operate, but to get it started, will need a surge watt of 2000. Your generator has to be able to produce that. I bought a 5000 watt generator and it runs 2 refrigerators and 2 aquariums.