What size generator for a sump pump?

Small motor, outsized stakes. The sump pump is the load that is cheapest to misunderstand and most expensive to lose.

Quick sizing question: what size generator for a sump pump.

What makes a sump pump a sizing question is not a watt table on this page. The power fails when the pit is filling. Read the nameplate, then the sourced page.

Where the watts are

This page does not convert horsepower into watts. Honda and Champion rows for sump pumps are on the sump pump sizing page. Startup is a motor start under water. It can be gentler than a well pump and still stall a generator that is already full.

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

The storms that kill grid power are the same storms filling the pit. The pump stops, the pit rises, and water can reach the basement floor during that storm. A flooded basement ruins flooring, drywall, and stored belongings, and the mold can outlast the storm. A fridge that stays closed is a different problem. The sump is the load where waiting to start the generator has a wet deadline.

It also keeps running after the rest of the outage feels handled. Later, when you have stopped thinking about the generator, the pump is still cycling. That is why it belongs on the first tier of a backup plan, on the transfer switch, not on a cord you can forget.

Sizing the generator around the pump

The pump rarely runs alone. The question is the pump plus everything else you will have on:

• Sump pump only: size the surge to the nameplate, then check the sourced page.
• Sump plus fridge, freezer, and lights: add those running watts in the calculator. Count only the largest surge.
• Sump plus well pump, furnace fan, and lights: the well pump is often the largest surge. See the well pump wattage guide for the locked-rotor trap.
• Whole house plus central air: the sump rides along. The air conditioner sets the class. See the whole-house sizing guide.

Build the list in the generator size calculator.

Surge discipline: pumps start under load

A sump pump starts because the float switch tripped. The impeller spins up against standing water whether or not the generator has spare capacity. If the generator is already near its limit, that start can stall it. Three fixes, cheapest first: shed other loads so the pump has surge room; size the generator with the headroom you set on the calculator; or add a soft-start kit.

Fuel: the sump cycles

The pump idles between cycles, so it is not the fuel problem. Air conditioning and an electric water heater are. Keep the sump powered and run the hungry loads on purpose. The calculator prints fuel from the list you enter. This page does not state a burn rate.

As an Amazon Associate I earn from qualifying purchases. The Amazon links below are affiliate links; they don't change the math.

Batteries vs generator for the sump

A battery inverter works if its surge rating clears the nameplate startup and the bank can deliver that burst more than once. The battery bank sizer does the runtime math. Batteries cover a shorter outage than a fueled generator. Many flood-prone homes use both: the battery first, the generator when the outage lasts. A fridge-only comparison that uses this site's battery-tab factors, rather than a pump surge, is on generator vs battery backup.

FAQ

What size generator do I need for a sump pump?

Read the nameplate, then the sourced sump-pump page. A generator chosen on running watts alone misses the startup. Add the pump's surge to the load plan in the calculator, along with everything else that will be on.

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

This page does not print a watt figure for that pump. The sourced sump-pump page cites the manufacturer rows. The startup still counts as a motor start when the pump kicks on under water, even if it is gentler than a well pump.

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 the sump pit. Power fails, the pump stops, and the pit can reach the floor during that same storm. It is the household load where shedding it causes direct property damage, so it goes on the backup list first.

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

Yes, if the inverter's surge rating clears the startup on the pump nameplate and the battery can deliver that burst. A dedicated battery backup sump pump is another route. Its runtime is shorter than a fueled generator. For a long outage, a generator plus the pump is the robust answer.

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

Longer than a load that runs continuously, because the pump idles between cycles. Fuel for that pattern is what the calculator prints after you enter the nameplate. This page does not state a gallons-per-hour figure.

Does a 240 V sump pump need a special generator?

Check the nameplate voltage and use a generator outlet that matches it. A transfer switch or a cord rated for that plug is the connection. Do not assume the pump matches the generator without reading the plate.