What size generator do I need for a sump pump?

A sump pump matters most in exactly the storm that knocks your power out. The good news: most sump pumps plug into a normal 120 V outlet. A portable generator and a heavy-duty cord can keep one going.

How many watts does a sump pump need?

It depends on the pump's horsepower (HP). Most homes have a 1/3 HP or 1/2 HP pump. Generator makers' charts give these figures:

Pump size	Running watts	Starting watts	Source
1/3 HP	500–800	1,000–1,600	Champion
1/3 HP	750	1,500	Honda
1/2 HP	800–1,050	1,300–2,150	Champion
1/2 HP	1,000	2,100	Honda
1 HP	2,300	4,500	Honda

Our calculator uses 1,000 running and 2,150 starting watts for a 1/2 HP pump. That's the top of Champion's starting range, so the pump will start.

Why the big jump at start-up? The motor has to go from standing still to full speed against a column of water. For a second or two it pulls about twice its running power. Briggs & Stratton's guide says motors can need "two to three times" their running watts to start.

What size generator for a sump pump and a fridge?

Most people want the fridge on too. Here's the math:

• Running: 1,000 (pump) + 400 (fridge) = 1,400 W
• Biggest surge: the pump, 2,150 − 1,000 = 1,150 W
• Starting: 1,400 + 1,150 = 2,550 W
• With 20% headroom: about 1,700 running and 3,100 starting watts

That's roughly a 3,000–3,500 watt generator. Add lights, Wi-Fi and phone chargers and you're still under 1,600 running watts. The calculator above is set up with this pair, so tap to add more.

An older fridge can need up to 700 running and 2,200 starting watts, going by Honda's chart. If yours is 15 or more years old, check its label.

Will a 2,000 watt generator run a sump pump?

• 1/3 HP pump: usually yes. At 1,000–1,600 W to start, it fits under 2,000 W.
• 1/2 HP pump: it's close. The start-up can reach 2,150 W, which is right at or just over a 2,000 W generator's peak. Turn everything else off while the pump kicks on.
• Pump plus fridge: not reliably. If both motors start at once, the generator will trip.

Check your pump's label before you rely on a 2,000 W unit. See what a 2,000 watt generator can run.

What size generator for 2 sump pumps?

Two 1/2 HP pumps need about 2,000 running watts together. If only one starts at a time, the starting total is 2,000 + 1,150 = 3,150 W.

But pumps switch on by themselves when the water rises. In a heavy storm, both can start in the same moment. To be safe, count both surges: 2,000 + 1,150 + 1,150 = 4,300 starting watts. That points to a generator of about 4,500–5,000 W.

Check your own pump's label

Chart numbers are averages. Your pump's label is better.

1. Unplug the pump first. Then read the label through the pit lid, or lift the pump out.
2. Find the HP and the amps. They're on a sticker or cast into the motor housing.
3. Multiply amps by 120 to get running watts. A pump rated 9.5 A uses about 1,140 W.
4. For starting watts, use two to three times the running watts if the label or manual doesn't say.

Many pumps use a "piggyback" plug for the float switch. Plug that same plug into the generator cord. Don't change how the float is wired.

How to connect it safely

• Use a heavy-duty outdoor extension cord rated for the load. A 12-gauge cord handles a 1/2 HP pump comfortably.
• Keep the generator outside, and don't run the cord through a window or door that lets exhaust drift in.
• Never backfeed the house through a wall outlet or dryer outlet. It can electrocute a utility worker and start a fire. If you want the pump on its home circuit, have a licensed electrician install a transfer switch or interlock. See interlock vs transfer switch.

How long will a generator run a sump pump?

A sump pump doesn't run all the time. It cycles on when the water rises. So a generator often sits well below full load, which stretches your fuel. Run time depends on the generator's tank and how busy the pump is. Our 5 gallons of gas guide shows how to estimate it.

Could a battery backup be enough?

For one 1/3 HP pump, a battery backup pump or a portable power station can bridge a short outage. Check that the power station's surge rating is above the pump's starting watts. A busy pump drains a battery fast, though. For outages that last days, a fuel generator is the safer bet.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.