Running vs starting watts: how to size a power station or generator

Every power station and every portable generator has two output limits. One is what it can deliver continuously. The other is what it can deliver for a moment while a motor starts. Sizing comes down to checking your appliances against both, and the second check is the one people miss.

What the two numbers are called

Makers do not use the same words. The continuous figure is called rated, running or continuous output; the short-term figure is called surge, peak, starting or max. The table quotes the wording each maker prints, from the spec pages linked on our cards.

Two things follow. First, “rated” on a station and “running” on a generator mean the same thing for sizing: the load it can carry indefinitely. Second, labels are not always clean. Generac's web spec table for the iQ3500 shows “Starting Watts: 0”, an error the maker's PDF spec sheet does not repeat; the card uses the PDF. On dual-fuel generators both figures drop on propane — the WEN DF680iX lists 4,500 W rated and 6,000 W surge on propane — so check the row for the fuel you will use.

Why motors need a starting surge

A heater or a kettle draws roughly the same power from the moment it is switched on. A motor does not. A stalled motor has no back-voltage to limit its current, so for a short moment it pulls far more than it will once it is spinning. Compressors in refrigerators, freezers and air conditioners, and the motors in sump and well pumps, all behave this way. Generac's owner's manual puts it as “some electric motors, such as induction types, require approximately three times more watts of power for starting than for running. This surge of power lasts only a few seconds” (Generac GP Series owner's manual, p. 17).

The appliance figures on this site show the same pattern. A top-freezer refrigerator runs at 360 W from its spec sheet; its starting figure, 1,200 W, comes from a generator maker's wattage chart because the fridge maker publishes no locked-rotor current. A 1/2 HP well pump runs at up to 1,000 W, and its locked-rotor current from Franklin Electric's motor table works out to 7,056 W. A 1,500 W space heater has no starting figure at all: there is no motor to start.

If the start needs more than the station or generator can deliver, the motor does not get the current it needs, however comfortable the running figure looked on paper.

The sizing rule: start the largest motor, run everything else

You do not add every appliance's starting watts together. Two motors rarely start in the same instant, so the worst realistic moment is one motor starting while everything else is already running. Generac's manual gives the rule in two steps: “1. Calculate the watts needed to start the largest motor. 2. Add to that figure the running watts of all other connected loads” (Generac GP Series owner's manual, p. 17). Its sizing guide writes the same idea as “Total kW Required = (Sum of all running wattages) + (Highest single starting wattage burst)” (Generac sizing guide).

This site's calculator does the arithmetic as running total plus the single largest step from running to starting among your appliances, which gives the same number. The two checks are then:

• Running check: your running total must not exceed the station's rated output, or the generator's running watts on your fuel.
• Starting check: running total plus the largest motor step must not exceed the station's surge/peak figure, or the generator's starting watts.

The full method is on the methodology page.

Worked example: fridge, sump pump and router

Take three appliances from our data: the top-freezer refrigerator (360 W running, 1,200 W starting), a 1/3 HP sump pump (400 W running, 1,600 W starting) and a Wi-Fi router (42 W, no motor).

• Running total: 360 + 400 + 42 = 802 W.
• Largest motor step: the sump pump, 1,600 − 400 = 1,200 W (the fridge's step is 840 W).
• Starting load: 802 + 1,200 = 2,002 W.

Now check two stations against it, using their published figures:

The AC70 has 198 W of running headroom, so a quick look at the running figure alone says yes. The starting check says no: when the pump kicks on with the fridge and router running, the load reaches 2,002 W against a 1,500 W surge rating. Take the pump out and the picture changes — fridge plus router is 402 W running and 1,242 W starting, which fits under the AC70's 1,500 W.

Had you added all three starting figures (1,200 + 1,600 + 42 = 2,842 W) you would overstate the requirement by 840 W, which can push you toward a larger and more expensive unit than the rule requires.

Fitting is only the first question. How long the station lasts is a separate one: the site's math puts this load at about 2,936 Wh per day if the pump runs two hours a day (the calculator's default), against about 910 Wh usable from the Jackery's 1,070 Wh battery (see station runtime). Try your own mix in the calculator.

Common mistakes

Reading the headline number as continuous

Generators are usually named by their starting figure. The WGen9500DFc carries 9,500 W running and 12,500 W starting on gasoline, and 8,500 W / 11,200 W on propane. Honda's April 2025 press release calls the EU7000iS's 7,000 W “rated power”, while Honda's own spec table lists 7,000 W max. and 5,500 W rated; our card uses the spec table.

Forgetting the refrigerator

A fridge looks small on a daily-energy basis — the top-freezer model in our data averages 47 W over a year from its annual-energy figure — but its compressor cycles on and off, and every time it comes back on it is a motor start. If the fridge is on the same station as a pump or a microwave, its running watts belong in the running total even when it seems idle, and its start may be the largest step if nothing bigger is connected.

Counting boost modes as output

Several makers offer a mode that runs heating appliances above the rated output: EcoFlow's X-Boost (the DELTA 3 lists “2200W (surge 3600W) with X-Boost” on top of 1,800 W rated), Anker's SurgePad and Bluetti's Power Lifting Mode, which the AC180 page ties to resistive loads up to 2,700 W. These are not continuous ratings and not starting ratings. Our fit test ignores them; see how fit is decided.

Treating a peak-draw figure as a motor start

Some devices publish a peak power draw rather than a starting surge. ResMed lists the AirSense 10 AutoSet at 53 W typical and 104 W peak. That peak is what the device can pull, not a motor start, but it still has to fit under the station's output. For any medical device, confirm the power figures with the device manufacturer.

Assuming a missing surge figure means “plenty”

When a maker prints no surge or peak figure, the starting check cannot be done. That is a gap in the data, not a pass.