How this calculator works
A generator has two ratings. The running, or rated, watts is what it can supply continuously, and the starting, or surge, watts is what it can supply for a few seconds while a motor starts. Add the running watts of everything you intend to power at once and compare that with the running rating. Add the single largest starting surge to the running watts of the other appliances, because motors rarely start at exactly the same moment, and compare that with the surge rating. The safety margin here is applied to the total starting load, which is the more conservative approach.
Formula
total running watts = sum of the running watts. total starting watts = sum of the starting watts. generator size = total starting watts x (1 + margin). current = generator size / voltage.
Worked example
A refrigerator rated 700 W with a 2,200 W surge, a microwave at 1,000 W with no surge and a furnace fan at 800 W with a 1,200 W start total 2,500 running watts and 4,400 starting watts. With a 20 percent margin, shop for a generator rated at about 5,280 W. At 120 volts that is about 44 amps.
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Frequently asked questions
Should I add every starting surge together?
In practice motors start at different moments, so the usual advice is to size for the total running load plus the largest single surge. This page adds every surge and then a margin, which gives a safer, larger figure; reduce the margin if the generator would otherwise be oversized.
Why do motor loads have a surge at all?
A motor draws a large current for a few cycles while it spins up. Refrigerators, freezers, furnaces and pumps are the usual culprits, and their labels state a locked-rotor or starting figure.
Are watts the same as volt-amps?
For resistive loads such as heaters and incandescent lights they are equal. Motors and electronics have a power factor below one, so their volt-amps are higher than their watts. A generator rated in watts can be overloaded by a load with a low power factor.
What about running a generator outdoors?
Petrol and propane generators must run outdoors, well away from windows and doors, because the exhaust contains carbon monoxide. Never run one in a garage, basement or attached shed, even with the door open.