What is the 80% rule for generators?

The 80 percent rule for generators says continuous load should stay at or below 80 percent of the unit's rated output. A 10,000 watt generator is treated as an 8,000 watt machine for anything that runs steadily.

The reserved 20 percent is not wasted. It absorbs motor starting surges, which hit three to six times running current, and it keeps the engine out of the wear zone where sustained full load shortens its life.

Portable units on hot days need the margin even more, since output falls with temperature and altitude.

Sizing backwards works too: total your real running load, divide by 0.8, and that is the smallest rating worth buying.

The standby generator sizing calculator applies the 80 percent factor and the surge math for you.