Generator Load Calculator: Running & Peak Starting Watts

Sizing a backup generator comes down to two numbers: the steady running watts of everything you want to power, and the higher momentary surge when a motor starts. This worksheet adds up the running watts of each appliance you list, then adds the single largest starting surge on top, giving the peak wattage the generator must supply. It is a planning tool; a licensed electrician should size and connect any generator, transfer switch, or interlock to code.

Running watts are what a device draws once it is up and going. Starting watts, sometimes called surge or LRA watts, are the brief spike a motor needs to overcome inertia — a refrigerator, well pump, or air conditioner can pull two to three times its running figure for a fraction of a second. Resistive loads like lights, heaters, and toasters have no surge, so their starting and running watts are the same.

The key insight is that appliances rarely start at the same instant. The standard method, used in portable-generator manufacturers' sizing guides, is to total all the running watts and add only the largest single surge, because that is the worst moment the generator has to ride through. Add appliances as rows, enter each one's running and starting watts and quantity, and the worksheet shows your running total, the largest surge, and the peak demand to compare against a generator's rated running and starting watts. The default rows use typical appliance wattages from those manufacturer guides; replace them with your own equipment's nameplate figures.