Starting watts vs running watts: which number actually runs your house

Last reviewed July 2026.

Every portable generator carries two wattage numbers, and the box headlines the wrong one. Starting watts (also sold as peak, surge, or maximum watts) is a burst the machine can hold for a few seconds. Running watts (rated or continuous watts) is what it can deliver for hours. The model name and the number in front of the box are almost always the peak — a “13,000-watt” generator is typically a 10,500-running-watt machine wearing its surge rating as its name.

Why two numbers exist at all

The surge rating is not a scam by itself — it exists because motors are greedy at start-up. A refrigerator compressor, a well pump or an AC unit draws two to three times its running draw for the first moment while the motor spins up. The starting-watts figure tells you what the generator can absorb for those seconds. The running-watts figure is the budget everything has to live inside once things are humming.

The gap on real machines

From the makers’ own published specs (the same dataset behind our ranking): a DuroMax XP13000EH is 13,000 peak / 10,500 running — the name is 24% bigger than the machine. A Westinghouse WGen9500 is 12,500 peak / 9,500 running. A Champion “4000W” inverter is 4,000 peak / 3,000 running — a full third of the name evaporates when the surge ends. The gap varies by design, which is why our table shows running / peak side by side rather than letting either number stand alone.

How to size with the honest number

• Add up the running watts of everything you want on at once — that total must fit inside the generator’s running watts, with headroom.
• Then check the single largest starting surge you’ll ask for (the biggest motor — usually the fridge, pump or AC) on top of the other running loads. That combined moment must fit inside the starting watts.
• Appliance nameplates and manufacturer spec sheets list both figures; the generator maker’s own wattage worksheet is the right template. We publish no sizing rules of our own — the arithmetic above is just how the two published numbers relate.

Why we rank on cost per running watt

Price divided by peak watts flatters whichever machine has the most optimistic surge rating. Price divided by running watts compares what you are actually buying: sustained power. That is the headline column in the ranking — next to noise, runtime and power quality, because the cheapest running watt is usually the loudest one. Every figure links back to the maker’s published spec page.

Generator Score indexes manufacturers’ published specifications and ranks on arithmetic; we test nothing and give no safety advice. Never run a generator indoors, in a garage, or near openings — carbon monoxide is deadly. A CO shutoff is a backstop, not permission: see what the sensor changes and the CO-shutoff record.