What Size Generator Runs a Sump Pump?

1. Generator Size by Sump Pump Horsepower

The half-horsepower row covers most residential installations. Run on its own during an outage, a small 3,500 watt generator handles it comfortably. Run alongside a refrigerator, freezer, furnace blower and lighting — which is how people actually use a generator — a 5,000 to 7,500 watt machine is the realistic target.

2. Working the Numbers for a Real Household

The correct method is to total the running watts of everything you want on, then add only the single largest start-up surge, because motors do not all start at the same instant.

A typical essentials list

Running total 3,750 watts. The largest surge belongs to the sump pump at 1,550 above its running draw, giving a peak demand of about 5,300. Adding twenty percent headroom — because generators should not sit at their rated limit — brings you to roughly 6,400 watts, so the next standard size up is 7,500.

That is the arithmetic behind the very common recommendation of a 7,500 watt portable for household backup. It is not arbitrary; it is what a standard essentials list including a sump pump actually works out to.

If the Sump Pump Is All You Need

Some households genuinely only want the sump pump covered — a basement that must not flood, with everything else able to wait. A 3,500 watt generator running a half-horsepower pump alone is entirely adequate, and small inverter generators in that range are quiet enough to run through the night without disturbing anyone.

3. The Hard Part: Connecting a Hardwired Pump

Sizing a sump pump generator is easy. Getting power to the pump is where plans fail, and it catches people out during the exact storm they prepared for.

Many sump pumps are hardwired into the panel or plugged into a dedicated receptacle in the pit area that is not conveniently reachable. Where the pump does plug in, a heavy-duty extension cord from a generator outside can work as a stopgap. Where it is hardwired, it cannot, and a transfer switch or interlock kit is required.

4. Battery Backup: The Complementary Answer

A generator is not the only way to keep a sump pump running, and for this particular appliance the alternative is genuinely worth considering alongside rather than instead.

Why Many Households Run Both

The gap a portable generator cannot cover is the first one. Power fails at three in the morning during a storm, and by the time someone wakes, dresses, goes outside and starts the generator, twenty minutes of inflow has gone unpumped. A battery backup pump covers exactly that window automatically, and then the generator takes over for the long haul once it is running.

For a finished basement in an area with a high water table, that combination is the belt-and-braces answer, and the battery unit is the cheaper half of it. If you have to choose only one and nobody is reliably home, the battery backup covers the specific failure mode that matters most.

5. How Long You Can Keep It Running

Sump pumps cycle rather than running continuously, which makes them cheap to keep alive over a long outage. The pump only draws power when the float switch calls for it, so a generator carrying a sump pump plus a few small loads is running at a fraction of its capacity most of the time.

• A mid-size portable typically runs 8 to 12 hours per tank of gasoline under moderate load.
• A 20 lb propane cylinder gives roughly 5 to 8 hours on a portable.
• Light loading extends both figures, and a sump pump alone is a light load.
• Dual-fuel machines let you switch to propane when gasoline stations are closed or empty.
• Fuel degrades — stabilise and rotate stored gasoline, or keep propane, which stores indefinitely.

For a multi-day outage the constraint is fuel logistics rather than generator capacity. Propane is the more dependable option for this exact scenario, because cylinders keep indefinitely in the garage while gasoline degrades within months and filling stations are frequently shut during the regional outages when you need them.

Do Not Run It Indoors

Worth stating plainly because the temptation is real with a basement appliance: never run a generator in a basement, garage or any enclosed space, even with doors open. Carbon monoxide from portable generators kills people every year. Run it well away from the building with the exhaust directed away from doors, windows and vents, and fit working CO alarms inside.

6. Matching Pump Capacity to Actual Inflow

Sizing the generator is only half the question. The other half is whether the pump itself can keep up with what is coming in, because a generator that faithfully powers an undersized pump still results in a flooded basement.

Capacity falls as vertical lift increases, so a pump rated generously at a low head may deliver considerably less through a long discharge pipe rising to grade. If your pit fills faster than the pump empties it during heavy rain even on mains power, the pump is undersized and a generator will not change that.

Watch the Cycle Rate During a Storm

A useful diagnostic during heavy rain: time how often the pump cycles and how long it runs. A pump running almost continuously is at its limit, and any interruption — power loss, a stuck float, a partial blockage — will let the pit overtop quickly. That is worth knowing before you rely on it through a generator-powered outage.

It also matters for generator planning. A pump cycling constantly through a storm draws far more average power than one cycling occasionally, and it presents its start-up surge repeatedly rather than now and then, so the generator sees a harder duty than a simple wattage calculation suggests.

7. A Storm Season Checklist

Sump pump backup is the one system where an untested plan is close to no plan at all, because it is called on precisely when everything else is going wrong. This is worth running through once a year.

• Start the generator and let it run under load for a few minutes to confirm it is healthy.
• Transfer the circuits and confirm the sump pump actually runs on generator power.
• Pour a bucket of water into the pit to check the float switch operates freely.
• Check the discharge line is clear and not frozen, blocked or disconnected outside.
• Confirm you have fresh fuel, stabilised if it is gasoline, or full propane cylinders.
• Test the battery backup pump if you have one, and check the battery date.
• Check carbon monoxide alarms indoors and replace batteries.
• Confirm you know where the transfer switch is and how to operate it in the dark.

The Two Failures People Actually Hit

In practice two things go wrong far more often than generator capacity. The first is fuel: gasoline left in a machine from last season that will not start. The second is the connection: a sump pump on a circuit the transfer switch does not actually cover, discovered when it matters. Both are eliminated by a single annual test that takes twenty minutes.