What size generator do I need for a well pump?
By pump horsepower, with and without the rest of the household sharing the generator — plus the 240 V and locked-rotor traps that send pumps back.
Generator size by pump horsepower
| Pump size | Running watts | Typical startup | Generator alone | With household loads |
|---|---|---|---|---|
| 1/3 HP | 750 | 1,500 | 2,000–2,500 W | 3,500–4,000 W |
| 1/2 HP | 1,000 | 2,100 | 2,500–3,500 W | 4,000–5,000 W |
| 3/4 HP | 1,500 | 3,000 | 3,500–4,500 W | 5,000–6,500 W |
| 1 HP | 2,000 | 4,000 | 4,500–6,000 W | 6,500–8,000 W |
"With household loads" assumes a fridge (~800 W running / 1,200 W surge), lights, phone charging, and a furnace fan (~1,100 W total running) sharing the generator — the standard outage kit.
Why surge, not running watts, picks the generator
The pump is the largest single surge in a home backup plan. A generator can run near its rated watts continuously but must clear the biggest startup spike on top of everything already running. For a 1/2 HP pump that spike is 2,100 W typical — but the locked-rotor (LRA) worst case on the motor plate is 3,200 W. Size to the LRA figure if the plate lists it, especially with other loads on the generator at start time. Full wattage table on our well pump watts guide.
Two traps that send generators back
No 240 V outlet. Most deep-well submersibles are 240 V. A 120 V-only generator — common in the 2,000 W inverter class — won't start the pump at any wattage. Look for an NEMA L14-30 or 14-50 receptacle; nearly all 3,500 W+ dual-fuel portables have one.
Stall on startup. If the generator bogs when the pump kicks in, the surge budget was exceeded. Fixes, cheapest first: staged startup (pump alone first, add loads after), a pump soft-start kit (~$100–200, cuts startup draw roughly in half), or upsize the generator.
The pick
Well water home, pump only or pump-first: a 4,500 W dual-fuel portable — clears a 1/2 HP pump's LRA with household loads running, has 240 V, and runs 9+ hours at partial load. 1 HP pump or pump + central AC: 8,000 W class, or add a soft-start kit to the biggest motor. Run your exact load list through the generator size calculator — well pump presets are built in.
FAQ
Can I run my well pump off a battery/inverter instead?
Yes, but the pump sizes the system: you need an inverter whose surge rating clears the pump's LRA — a 3,000 W surge inverter for a 1/2 HP pump. The battery bank sizer handles the bank side once you know the surge inverter.
How long will a generator run a well pump per day?
A household of four uses 200–300 gallons/day; a 10 GPM pump needs under an hour of total run time — about 1 kWh of energy. One gallon of gas covers days of pumping. Cycle it morning and evening rather than idling all day.
The 4,500 W dual-fuel class is the workhorse for well-water homes: enough surge for the pump, 240 V on board, quiet at partial load.