A sump pump fails during a power outage at the worst possible moment — when the storm that knocked out the power is also filling your basement. Here's exactly what size generator you need to keep it running.
The cruel irony of sump pump outages: they fail precisely when you need them most — during the heavy rain storms that also knock out power. A generator-backed sump pump is one of the highest-value backup power investments you can make. And the good news is that sump pumps don't require massive generators — you just need to match the motor's starting surge.
💡 Running the sump pump plus other appliances? Use our free generator calculator to add all your loads and get a precise total recommendation.
Sump pumps are rated in horsepower (HP). The HP rating directly determines watt requirements. Most residential sump pumps are 1/3 HP or 1/2 HP. Here are the real numbers:
| Sump Pump Size | Running Watts | Starting Watts (Surge) | Min. Generator Size |
|---|---|---|---|
| 1/4 HP | 400–600 W | 900–1,250 W | 2,000 W |
| 1/3 HP | 600–800 W | 1,300–1,700 W | 2,000 W |
| 1/2 HP | 800–1,050 W | 1,800–2,150 W | 2,500 W |
| 3/4 HP | 1,000–1,400 W | 2,400–3,000 W | 3,500 W |
| 1 HP | 1,500–2,000 W | 3,000–4,500 W | 5,000 W |
Find your pump's HP: Check the nameplate on the pump motor. It will show HP and amps — multiply amps × 120V to confirm running watts. Most submersible sump pumps in homes built after 1990 are 1/3 HP or 1/2 HP.
A sump pump motor draws 2–3× its running wattage for 1–2 seconds when it starts. This happens every time the float switch triggers. During a heavy storm, the pump may cycle on every 5–10 minutes — meaning the starting surge hits frequently, not just once.
If your generator's surge/peak wattage is below the pump's starting demand, the generator will either trip its overload breaker or fail to start the pump at all. The pump sitting in the pit while the generator is "running" is a common failure mode — the generator runs, but the pump never starts because the surge exceeded capacity.
The fix: size to the pump's starting watts, not running watts. Always add the starting surge to whatever other loads are running at the moment the pump kicks on. A refrigerator compressor cycle and a sump pump start at the same time would demand the combined surge of both. See our sizing guide for the math.
If your only goal is to keep the basement dry and you'll manage everything else with flashlights and patience, a 2,000W generator covers a 1/3 HP pump. For a 1/2 HP pump, use a 2,500W generator to maintain surge margin.
Sump pump (1/2 HP): 900W running, 2,150W starting
Refrigerator: 175W running, 1,200W starting
Lights + chargers: 200W running
Total running: 900 + 175 + 200 = 1,275W running.
Peak (both motors starting simultaneously, worst case): 1,275 + 2,150 + 1,200 = 4,625W peak.
Recommended: 5,000W generator (typically 6,250W surge). This covers the worst-case scenario of both motors starting at the same time.
Add a 10,000 BTU window AC (1,100W running / 2,500W starting) to the above scenario. The AC surge is now the largest single starting surge. Peak demand: 1,275 + 1,100 + 2,500 = ~5,000W peak. A 7,500W generator handles this setup with comfortable headroom.
An alternative worth knowing: dedicated sump pump battery backup systems (like the Zoeller Aquanot or Wayne ESP25) provide 5–12 hours of pump operation on a built-in battery without any generator. These are excellent for short outages but have limited runtime.
Use a battery backup sump pump when: outages are typically under 4 hours, you don't want the noise/maintenance of a generator, and you don't have other critical loads to back up.
Use a generator when: outages regularly exceed 4–8 hours (common in ice storms and hurricanes), you need to power other appliances simultaneously, or you have a 3/4 HP+ pump a battery system can't handle.
Clean power, quiet operation, and enough surge for a 1/3 HP pump. Lightweight enough to store in a basement utility closet for quick deployment during storms.
→ Browse 2,000W Inverter Generators on Amazon
The most popular size for homeowners who want to protect the basement AND keep the refrigerator running. Electric start, dual-fuel models add flexibility during extended outages.
→ Browse 5,000W Home Backup Generators on Amazon
Handles sump pump, refrigerator, window AC, lights, and more. Dual-fuel capability (gas + propane) is a lifesaver when gas stations run out during regional storm events.
Yes — for a 1/3 HP sump pump (1,300–1,700W starting), a 2,000W generator (typically 2,200W surge) is sufficient. For 1/2 HP pumps (1,800–2,150W starting), the margin is thin. Use a 2,500W generator for 1/2 HP pumps to avoid tripping the overload during startup, especially if anything else is running simultaneously.
Yes — sump pumps are simple induction motors that work fine on generator power, both conventional and inverter types. Connect via a heavy 12 AWG outdoor extension cord or through a transfer switch to power the sump pump's dedicated outlet. Never run the generator in your basement or garage — CO2 poisoning risk is real and fast.
Two things typically happen: the generator's overload breaker trips (shutting off power to everything), or the motor fails to start (humming but not spinning), which can burn out the pump motor over time. Either way, your basement floods. Don't undersize — add the pump's starting watts to your total load calculation.
A generator can run a sump pump indefinitely — as long as you have fuel and follow the maintenance schedule (oil changes per the manual). Most 5,000W generators run 8–12 hours per tank at ~50% load. For multi-day outages, have extra gasoline in approved containers, or use a dual-fuel generator and a propane tank.
Add your sump pump and all other appliances for an exact generator size.
Running and starting watts for pumps, HVAC, kitchen appliances, and more.
Well pump wattage and sizing — similar to sump pumps but often larger.
Complete outage prep guide including what to prioritize and what size to buy.