Size your backup power for the sump pump before anything else in the house. Per our generator sizing tool, a half-horsepower pump draws about 800 watts running and 1,300 watts starting — and that running figure alone is larger than the 600-watt spike a refrigerator makes at startup.
Everything else on your list is a rounding error next to it.
The numbers, side by side
These are the figures our sizing tool publishes, and they're the ones the tool itself does the arithmetic with:
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator | 150 W | 600 W |
| Sump pump (1/2 HP) | 800 W | 1,300 W |
| Well pump (1/2 HP) | 750 W | 1,400 W |
| LED lighting, eight bulbs | 70 W | 70 W |
| Phone charging | 30 W | 30 W |
Read the fridge row and the pump row together and the point lands: a battery that comfortably rides out a refrigerator's startup can be flattened by a sump pump doing nothing unusual. If you want the concept behind that gap, surge watts vs running watts is the post — this one is about the appliance that punishes it hardest.
Runtime is the wrong question
The natural question is "how many hours will a battery run my pump," and it has no useful answer, because a sump pump doesn't run continuously. It sits idle until the float rises, works hard for a short burst, and shuts off. How often that happens depends entirely on how much water is arriving — which depends on the storm, your soil, your grading, and your neighbor's downspout.
Time it yourself
If you want a real number for your own house, time it during the next heavy rain: count how many times the pump kicks on in ten minutes, and how long each run lasts. That is the only duty cycle that matters, and it costs nothing but a wet afternoon at the window.
Cycles, not hours
In a slow drizzle a pump might run for a few seconds every several minutes. In a genuine storm it can run nearly constantly, and that's precisely when the power is most likely to be out. Assume the bad version when you plan, because the bad version is the correlated one.
Two different products
Worth being clear, because the words overlap:
- A battery backup sump pump is a second pump, plumbed into the same pit, with its own battery and float. It takes over automatically when the main pump has no power. It is a plumbing job.
- A power station running your existing pump is an electrical workaround — you plug the pump into a battery. It's flexible, and it depends on somebody being home and awake to do it.
The first is what you want if your basement flooding badly is a real prospect. The second is what many households actually have, and it's far better than nothing — provided it can clear 1,300 watts on startup, which rules out the smaller units.
Where this falls short
We publish the half-horse figure and not the third-horse one, so this post doesn't carry a 1/3 HP row. A smaller pump draws less, but we aren't going to put a number on your pump when the real one is printed on its own nameplate. Go and read it — it takes a minute, and it's the only figure that's actually about your house.
A pump is also the wrong place to start if water is getting in faster than it should. Gutters that discharge next to the foundation, grading that slopes toward the house, and a downspout that ends six inches from the wall are cheaper problems to solve than an electrical one. The flooding guide covers that side.
And nothing here helps if the pit is silted up or the float is stuck. A pump that hasn't been tested since it was installed is a pump with an unknown opinion about whether it still works. Pour a bucket of water into the pit twice a year and watch it come on.
Where to go next
Put your real appliance list into the generator sizing tool — with the pump in the list, the number it gives you'll be higher than you expect, and that's the point. Then what to do in a multi-day outage covers the rest of the household while the pump is eating the budget.