The pump that has to start every time
A sump pump matters most exactly when the power goes out: in heavy rain.[1] It switches on and off by itself as the pit fills, and every start is a motor start. Your generator has to handle that surge on top of whatever else is running, at any moment.
That’s why a sump pump is always “must run together” in the calculator — you can’t schedule it.
Check the horsepower
Most homes have a 1/3 or 1/2 HP pump; the numbers here are for 1/2 HP. A 3/4 HP pump needs noticeably more. The motor label gives amps — multiply by 120 V for running watts.
Sump pump plus the rest of the house
Motors don’t all start in the same second, so you only add the single biggest startup jump on top of everything that’s running.
- Sump pump + fridge: 1,750 W running, 3,950 W starting → at least 2,188 W rated (mid portable). Open in calculator
- Sump pump + fridge + lights + Wi-Fi: 1,870 W running, 4,070 W starting → at least 2,338 W rated (mid portable). Open in calculator
We divide the running load by 0.8 because a generator shouldn’t sit above about 80% of its rating for hours.
Questions
Can a small inverter run a sump pump?
Only if its starting rating covers the pump’s surge. Check the peak watts, not just the running watts.
Should I get a battery backup instead?
A battery backup pump, which only turns on if the main pump fails, is a good second line of defense.[2] A generator keeps the main pump and the rest of your essentials running for longer outages.