How this calculator works
The calculator uses the same method that generator manufacturers describe in their sizing guides:
- It adds the running watts of everything you tick.
- It finds the item with the largest gap between its starting and running watts, and adds that gap once. Motors need a brief surge to start, but they rarely all start at the same instant.
- It adds about 20 percent headroom to the running total, because a generator working at its limit runs hot, burns more fuel and has nothing in reserve.
- It rounds up to the next 500 watts and suggests the class of generator that usually covers that figure.
The fuel estimate assumes your loads cycle on and off so that the average draw is about 60 percent of the running total, and uses typical fuel consumption per kilowatt-hour for gasoline, propane and diesel engines. Treat it as a rough planning figure.
Where the numbers come from
The wattages in the list are typical planning values in line with commonly published appliance wattage charts. Real appliances vary, sometimes by a lot: a modern refrigerator may draw less than the figure here, an old well pump more. The nameplate on the appliance, or its manual, gives the real figure. If it lists amps instead of watts, multiply by the voltage (120 or 240). Use the “Something else” row to enter your own figures.
Reading the result
- Look for at least X running watts: compare this with the generator’s rated or running watts, not the big peak number on the box.
- Starting watts to allow: the generator’s starting, surge or maximum rating should meet this figure.
- Fuel per day: a rough range for the hours per day you selected. Our guide to how much gas a generator uses explains the arithmetic.
Our article on what size generator you need walks through a worked example, and our portable generator guide explains the classes the calculator suggests. Whatever size you choose, read generator safety before the first outage.
