What Size Generator Do I Need for a Well Pump? Starting Watts, 240V and Solar Options

How to size a generator for a well pump: running vs starting watts by horsepower, 120V vs 240V pumps, soft starters, and whether a solar generator can run your well in a blackout.

Short answer: as a rule of thumb, a typical 1/2 HP submersible well pump needs roughly 1,000 W running and up to around 2,000–3,000 W at start-up, while a 1 HP pump may need 1,500–2,000 W running and around 4,000–6,000 W to start. Most rural households land on a 5,000–7,500 W generator with 240 V output. Always check the pump nameplate, because these are planning ranges, not specs.

Why wells are the hard part of a blackout

If you are on a private well, no power means no water: no drinking, no flushing, no washing. Many people buy a generator big enough for the fridge and discover it cannot start the pump. Motors draw a surge of several times their running current for a second or two at start-up, and the generator must survive that surge.

Rough sizing by pump size

1/2 HP: ~1,000 W running, ~2,000–3,000 W starting.
3/4 HP: ~1,200–1,500 W running, ~3,000–4,500 W starting.
1 HP: ~1,500–2,000 W running, ~4,000–6,000 W starting.
1.5–2 HP: 2,000–3,000 W running, 6,000–8,000 W+ starting.

Older pumps, deep wells and 2-wire pumps (no separate control box) tend toward the higher end. Add whatever else will run at the same moment.

Check these on your pump

Voltage: most submersible pumps are 240 V. Your generator must provide 240 V, usually through an L14-30 outlet, and you need transfer equipment to feed the well circuit (see interlock kit vs transfer switch).
Horsepower and amps: on the control box or pressure switch paperwork.
Locked rotor or service factor amps: these tell you the start-up load. Multiply amps by volts for watts.

Tricks to start a pump on a smaller generator

Turn off every other load before the pump cycles on.
Install a soft starter on the pump control, which can cut surge significantly; check compatibility with your pump.
Use a larger pressure tank so the pump cycles less often.
Run the pump on a schedule to fill the tank and some storage containers, then shut it off.

Can a solar generator run a well pump?

Only the larger units. You need a 240 V-capable power station (or two linked 120 V units that support split phase) with surge capacity above the pump's start-up load, plus enough battery to do useful pumping. Many 2–3 kWh units with 120 V only cannot. A DC solar well pump is a more efficient long-term option for off-grid homes. Our solar generator sizing guide walks through the battery maths.

Store water anyway

Even with the right generator, keep stored water for the first day and for when the fuel runs out; see our emergency water storage guide.

Sources

Your pump's nameplate and manufacturer installation manual (Franklin Electric, Goulds, Grundfos and others publish starting amp tables).
US EPA, private wells after a power outage: epa.gov
CPSC portable generator safety: cpsc.gov

Frequently asked questions

What size generator runs a 1 HP well pump?

Plan on a generator that can handle roughly 4,000 to 6,000 W of surge, commonly a 5,000 to 7,500 W unit with 240 V output.

Will a 3500 watt generator run a well pump?

Often a 1/2 HP pump, sometimes a 3/4 HP, if nothing else is running. Many 3,500 W units are 120 V only, which will not run a 240 V pump.

Can I run a well pump on a solar generator?

Only on larger 240 V-capable units with high surge rating. Check the pump's starting amps first.

What is a soft starter for a well pump?

A device that ramps up motor current to reduce start-up surge, which can let a smaller generator start the pump.