Running vs Starting Watts for Generators
Why motors surge on startup, how to add up a realistic peak load, and how much headroom to leave so a generator runs safely for years.
The two numbers on every motor
Anything with an electric motor carries two wattage figures, and mixing them up is the most common sizing mistake. Running watts is the steady power the device draws once it is up to speed. Starting watts, sometimes called surge or locked rotor watts, is the much larger jolt needed to overcome inertia in the first fraction of a second. A refrigerator might run on 700 watts yet demand 2,000 to start, so a generator sized only on running watts will stall or trip the moment the compressor cycles on.
Building a realistic peak load
To size a generator you total the running watts of everything that will be on together, then add the single biggest starting surge on top. You add only one surge because motors almost never start at the same instant, and the honest worst case is your steady loads humming along when the hardest device kicks in. If two large motors truly must start together, treat their combined surge as the single figure. That total is the peak the generator has to survive without sagging or shutting down.
Headroom and continuous versus peak rating
Generators are advertised with a peak or surge rating and a lower continuous or rated output, and the continuous number is the one you should size against. Adding headroom, commonly around 20 percent, pushes your recommended figure comfortably below a unit's continuous rating. That cushion keeps the engine off its limit, reduces heat and noise, and leaves room for a load you forgot or add later. It also means the generator is not straining every time a motor surges, which extends its service life.
Things that lower real output
The wattage printed on the box assumes ideal conditions. High altitude thins the air an engine needs, and every 1,000 feet above sea level can cut output by roughly three percent. Hot weather, a dirty air filter, old fuel, and long or thin extension cords all shave usable power too. Because of this it is wise to keep some margin beyond the bare minimum, and to compare your calculated target against the generator's continuous rating rather than its optimistic peak.