Starting Watts vs Running Watts, Explained
Updated July 2026
Running watts is what an appliance draws while working; starting watts is the bigger gulp its motor takes for the first second. The gap is a factor of two to three on anything with a compressor or pump, it exists for about one second, and it is the number that actually sizes your generator. Miss it and the generator stalls the moment the fridge clicks on.
Where the spike comes from
An induction motor at standstill is nearly a short circuit: until the rotor spins up, the windings draw everything the line will give, a figure the motor's nameplate calls locked-rotor amps (LRA). Once the rotor reaches speed, impedance rises and the draw falls to the running figure. The whole event takes half a second to two seconds, longer for a motor starting against load, like a compressor with head pressure or a pump pushing a full column of water.
Resistive loads have no rotor, so they have no spike. A space heater, a coffee maker, a water heater element, an incandescent bulb: the nameplate number is the whole story, on at full draw from the first millisecond. Electronics with switch-mode supplies (TVs, laptops, LED lighting) have a tiny inrush measured in milliseconds that no generator notices.
Who spikes and by how much
| Appliance | Running | Starting | Ratio |
|---|---|---|---|
| Refrigerator | 700 W | 2,200 W | 3.1x |
| Chest or upright freezer | 500 W | 1,500 W | 3.0x |
| Sump pump, 1/3 HP | 800 W | 1,300 W | 1.6x |
| Sump pump, 1/2 HP | 1,050 W | 2,150 W | 2.0x |
| Well pump, 1/2 HP | 1,000 W | 2,100 W | 2.1x |
| Well pump, 1 HP | 2,000 W | 4,000 W | 2.0x |
| Furnace blower (gas heat) | 800 W | 2,350 W | 2.9x |
| Window AC, 10,000 BTU | 1,200 W | 2,400 W | 2.0x |
| Central AC, 3 ton | 3,500 W | 6,000 W | 1.7x |
| Washing machine | 1,000 W | 2,300 W | 2.3x |
Planning numbers from the manufacturer wattage charts, same data as the wattage picker. Your nameplate wins where it disagrees.
The sizing rule the spike creates
Add the running watts of everything on the generator at once. Then add the single largest (starting minus running) gap in the list, because starts are one-second events and your motors won't all pick the same second. That total is your peak demand. The generator needs a peak rating above it and a rated (running) figure above your running total with margin; we use 25%, which keeps the engine at or under 80% continuous, where the manuals want it.
Two edge cases earn exceptions. Twin sump pumps in a storm genuinely can start together; count both spikes. And a compressor that short-cycles (starts every minute or two) leans on the peak rating constantly, so give it more margin than a fridge that starts hourly.
Reading your own nameplates
Every motorized appliance has a data plate: amps times volts equals running watts. If it lists LRA, that times volts is your true starting figure, better than any chart. If it lists horsepower, a rough conversion for sizing is 1,000 to 1,200 running watts per HP for small motors, with the usual doubling or tripling at start. Ten minutes with a flashlight beats every table on the internet, ours included.
Questions people ask
Do starting watts matter if appliances start one at a time?
Yes, but only the biggest one. The standard method assumes exactly that: everything running, plus one motor (your largest) starting on top. What you avoid by staggering starts manually is only the rare double-start, which matters when two big motors share one small generator.
What ratio of starting to running watts should I assume?
Two to three times for compressors and pumps, with three the safe blind guess (a 1 HP well pump: 2,000 running, 4,000 starting). Fans and blowers run lighter, around two times. Resistive heat and electronics, one time: no spike worth counting. The nameplate LRA beats all ratios when you can find it.
Why does my generator handle the load on paper but still bog?
Usually voltage sag during a start. A start spike at the edge of the peak rating pulls the voltage down; everything else on the cord slows, and a second appliance’s overload may trip. Chart math treats the peak rating as a cliff edge; real engines want the spike to land inside it with room, which is what our 25% running-watts margin indirectly buys.