What size generator for a sump pump?

Small motor, outsized stakes. The sump pump is the cheapest load on your backup list to power and the most expensive one to lose.

Sump pumps are modest appliances. What makes them a sizing question isn't the wattage — it's that the power fails exactly when the pit is filling. Here are the numbers, then the sizing logic.

Sump pump wattage by horsepower

Figures from our calculator's published wattage presets (manufacturer and DOE/consumer-testing ranges). Running watts ≈ HP × 746 W ÷ efficiency (~75%) — sump pumps run slightly leaner than well pumps because they're short-lift, high-speed motors. Startup for sumps is roughly 1.6× running, gentler than the 2–3× of well pumps or the 4× of AC compressors.

Why the sump matters more in an outage than any other load

Follow the chain: the storms that kill grid power are the same storms dumping rain into your pit. Grid fails → pump stops → pit rises → water finds the basement floor, typically within the hour during heavy rain. A flooded basement means ruined flooring, drywall, stored belongings, and often a mold problem that outlasts the storm by months. Compare that to a fridge: lose it for four hours and you lose nothing but a little cold air. The sump is the one load where "we'll get the generator going in a bit" has a hard, wet deadline.

It's also the load that keeps working after everything else is handled. Two days into an outage, when you've stopped thinking about the generator, the pump is still cycling in the background. That duty profile matters for fuel math (below) — and it's why the sump belongs on the first tier of any backup plan, wired into the transfer switch, not on a plug-in extension cord you'll forget to run.

Sizing the generator around the pump

The pump alone barely registers — but it never runs alone. The sizing question is the pump plus everything else you'll have on:

• Sump pump only: a 2,000–2,500 W generator covers a 1/2 HP pump with wide headroom — even a compact inverter unit.
• Sump + fridge + freezer + lights: 800 + 150 + 200 + 80 ≈ 1,230 W running, with the sump's 1,300 W spike on top → about a 3,500 W generator after 20% headroom. This is the minimum most basement homes should own.
• Sump + well pump + furnace fan + lights: 800 + 1,000 + 700 + 80 ≈ 2,580 W running, well pump's 2,100 W startup as the biggest surge → 4,000–5,000 W class. See the well pump wattage guide for the LRA trap that sizing must clear.
• Whole house + central AC: the sump rides along; the AC is what sets the size. See the whole-house sizing guide.

Two homes, two lists — build yours in the generator size calculator; the ½ HP sump pump is one of its default presets.

Surge discipline: pumps start under load

A sump pump typically starts because the float switch tripped — meaning the impeller spins up against standing water, and the 1,300 W spike lands whether or not it's convenient. If your generator is already carrying near its limit, that spike can stall it right when it matters. Three fixes, cheapest first: shed non-essential loads so the pump always has surge headroom; keep the generator sized above peak demand + 20% (the calculator's method); or add a soft-start kit to cut startup draw roughly in half.

Fuel: the sump is cheap to keep alive

Sump pumps idle between cycles — a wet-basement night might mean 10–15 minutes of pumping per hour, an average draw of just 150–200 W. Using the 50%-load fuel rule (roughly 1 gallon of gasoline per 7,000 W of rated output per hour), even a small generator covers a full night of duty cycling on well under a gallon. The pump is not the fuel problem; the AC and water heater are. Keep the sump powered and you can run the hungry loads selectively.

Batteries vs generator for the sump

A battery/inverter backup works if the inverter's surge rating clears the pump's 1,300 W startup — a 2,000+ W surge inverter minimum for a 1/2 HP pump — and the bank can deliver that burst repeatedly. Our battery bank sizer does that runtime math. The honest comparison: batteries cover hours, a generator covers days, and outage-plus-flood scenarios are frequently days. Many flood-prone homes run both — battery for the first hour, generator for everything after.

FAQ

What size generator do I need for a sump pump?

A 1/2 HP sump pump needs 800 watts running / 1,300 watts starting; a 3/4 HP needs roughly 1,200 running / 2,000 starting. A small 2,000–2,500 watt generator runs a sump pump alone with headroom. If the pump shares the generator with a fridge, freezer, and lights, 3,500–4,000 watts covers it. Add the pump's surge to your total load plan, not to a generator chosen on running watts alone.

How many watts does a 1/2 HP sump pump use?

About 800 watts running and 1,300 watts at startup. That's smaller than a well pump of the same horsepower because sump pumps are high-speed, short-head motors that run more efficiently. The startup surge is only about 1.6× running — gentler than a well pump or AC compressor — but it still counts as your biggest starter when the pump kicks on under load.

Why does the sump pump matter so much in an outage?

Because outages and floods arrive together: the storms that knock out power are the ones filling your sump pit. Power fails → pump stops → pit fills → basement floods — often within an hour in heavy rain. It's the one household load where shedding it causes direct property damage, which is why it's first onto the backup list, not last.

Can I run a sump pump on an inverter or battery instead of a generator?

Yes, with a caveat: the inverter must clear the pump's 1,300 W startup surge, which means a 2,000+ W surge-rated inverter and a battery bank that can deliver it. A dedicated battery backup sump pump is another route, but its capacity is measured in hours, not days. For a multi-day outage, a generator plus the pump is the robust answer.

How long will a generator run a sump pump on one gallon of gas?

A long time, because sump pumps idle between cycles. A 1/2 HP pump cycling in a moderately wet basement might run 10–15 minutes per hour, averaging 150–200 W. On a generator at 50% load, roughly 1 gallon of gas per 7,000 W of rated output per hour — so even a small 2,500 W unit sips far less than a gallon over a full night of duty cycling.

Does a 240 V sump pump need a special generator?

Residential sump pumps are 120 V — any portable generator runs one directly, including through a transfer switch or heavy-duty extension cord. Only deep-well water pumps and larger whole-house loads routinely need the generator's 240 V outlets. Check the pump's nameplate, but nearly every pedestal and submersible sump in a home is a standard 120 V plug.