TL;DR
A 1/3 HP sump pump needs at least a 1,500W generator, a 1/2 HP pump needs 2,100W, and a 3/4 HP pump needs 3,500W. Always factor in the starting wattage surge — it can be 2-3x the running watts.
Why Sump Pumps Have Unique Power Requirements
Sump pumps have a high starting wattage because the motor needs extra power to overcome inertia and move water from a standstill. A 1/2 HP sump pump might draw only 800W while running, but it can spike to 2,100W or more when starting — especially if the pump is working against water pressure.
This starting surge is the critical factor in generator sizing. If your generator can't handle the starting surge, the pump won't start, and the breaker will trip. You need a generator rated to handle the starting watts, not just the running watts.
During a storm, your sump pump is the most critical appliance — without it, your basement floods. Always dedicate enough generator capacity to handle the pump's starting surge, even if you're also powering other devices.
Top picks for this size
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These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
Sump Pump Generator Sizing by HP
- 1/3 HP sump pump: 800W running, 1,500W starting → 1,500W generator minimum
- 1/2 HP sump pump: 800W running, 2,100W starting → 2,500W generator minimum
- 3/4 HP sump pump: 1,100W running, 2,800W starting → 3,500W generator minimum
- If you're also running lights, fridge, and a furnace fan: add their running watts and use the highest starting surge
Frequently asked questions
What size generator do I need to run my sump pump?
Yes. A 3,500W generator can run a 1/2 HP sump pump (2,100W starting surge), plus a refrigerator (600W starting), furnace fan (800W), and several lights. Just don't try to add a window AC or microwave at the same time.
Should I get a generator specifically for my sump pump?
If your sump pump is your primary concern and you have municipal water pressure (for toilets/showers), a 1,500-2,500W inverter generator is sufficient. These are lightweight, quiet, and can run the pump plus a few lights and the fridge.
What happens if my generator is too small for the sump pump?
The generator either trips its breaker when the pump starts, or the voltage drops low enough to damage the pump motor. This is why sizing for starting watts is critical — a generator that's 20% undersized won't work at all.