What size generator runs a sump pump? Wattage, surge, and keeping the basement dry

Power outages and heavy rain often arrive together. That's bad timing for any home with a sump pump, because the pump stops just when groundwater is rising. A generator sized for the pump, plus a simple plan, can be the difference between a dry basement and an expensive cleanup. This guide covers how much power sump pumps need, how to size a generator for them, and the alternatives worth considering.

How much power does a sump pump use?

Sump pumps are rated in horsepower. Typical residential sizes are 1/3 hp and 1/2 hp, with 3/4 hp and 1 hp pumps used where water tables are high. Approximate figures:

Pump size	Typical running watts	Typical starting watts
1/3 hp	about 800	about 1,300 to 2,000
1/2 hp	about 1,000 to 1,050	about 2,000 to 2,200
3/4 hp	about 1,300 to 1,500	about 2,500 to 3,000
1 hp	about 1,800 to 2,000	about 3,500 to 4,000

These are typical numbers. Your pump's label, or its manual, lists the actual amps. Multiply amps by 120 volts to get running watts. The starting surge is the bigger concern, because the pump's motor starts under load each time the float switch trips.

Sizing a generator for the pump

A sump pump starts automatically and without warning. That means your generator must be able to handle the pump's starting surge at any moment, on top of whatever else is running. To size it:

1. Add the running watts of everything else you'll run at the same time: refrigerator, lights, router, furnace blower.
2. Add the sump pump's running watts.
3. Add the larger of the sump pump's surge or the largest other motor's surge.

Example: refrigerator 700 W, lights and electronics 400 W, furnace blower 800 W, and a 1/2 hp sump pump at 1,050 W running gives a running total of 2,950 W. The largest surge is the pump or blower, about 1,100 to 1,500 W extra. You'd want a generator with at least about 4,500 starting watts and about 3,700 running watts for a 20% margin. A common 5,000 to 6,000-watt portable generator handles this comfortably.

If you also have a second pump, or both pumps could start together, add both surges.

Connecting the pump

Most sump pumps plug into a dedicated outlet near the pit, which makes them easy to run from a generator with a heavy-duty extension cord. Use an outdoor-rated cord of adequate gauge, often 12 AWG for a 1/2 hp pump on a long run. Route it so it isn't pinched by a door or window, which could also let exhaust into the home. A better long-term solution is a transfer switch or interlock with the sump circuit included, installed by an electrician, so the pump stays on its normal outlet. See transfer switch vs. interlock.

A storm-night plan

• Before the storm: test the pump by pouring a bucket of water into the pit and watching it cycle. Fuel the generator, check the oil, and lay out cords.
• When the power goes out: start the generator outdoors, at least 20 feet from windows, doors, and attached garages, as Ready.gov advises. Connect the pump first, then other loads.
• Through the night: check the pit level periodically. If the pump cycles every few minutes, the generator should run continuously. If it cycles rarely, you can manage other loads around it, but never unplug the pump while water is rising.
• Refueling: let the generator cool before refueling. If the pit fills fast, time refueling right after the pump finishes a cycle.

Alternatives and backups

Battery backup sump pumps. A secondary pump that runs from a deep-cycle battery when power fails or the main pump can't keep up. Capacity is limited by battery size, so check the rated gallons pumped per charge. Many homeowners pair a battery backup with a generator: the battery covers the first hours, or the time when you're asleep or away, and the generator recharges it.

Water-powered backup pumps. These use municipal water pressure to create suction and remove water from the pit, without electricity. They need adequate city water pressure, and they use a significant amount of water. Check local plumbing codes, which often require a backflow preventer. They aren't suitable for homes on private wells, because the well pump needs power.

Portable power stations. Large battery power stations can run a small sump pump for a limited time. Check that the station's surge rating exceeds the pump's starting watts, and calculate runtime from the battery capacity and the pump's duty cycle.

Maintenance that prevents emergencies

• Test the pump every few months and before storm season.
• Clean the inlet screen and make sure the float moves freely.
• Check that the discharge line outside is clear and drains away from the foundation.
• Replace aging pumps proactively. Many last roughly 7 to 10 years, depending on use.
• Keep a spare pump if your basement floods often. Swapping a failed pump during a storm is much easier than finding one in a store.

Flood and electrical safety

If water has already reached outlets, cords, or the generator, don't touch them. Ready.gov's flood guidance warns about electrocution risks in flooded areas. Turn off power at the main breaker only if you can do it safely, without standing in water, or call your utility. Keep the generator itself dry, on a raised, level surface outdoors. A wet generator or connected equipment can shock you.

Carbon monoxide reminder

Basements and attached garages are tempting places to run a generator during a storm, because they're dry. Don't. The CDC reports that more than 400 Americans die each year from unintentional, non-fire CO poisoning, and it names portable generators as a source. Use a generator cover or tent designed for running units, keep the generator outside, and keep battery-powered CO alarms working inside.