On this page
| Pump size | Running watts | Starting surge |
|---|---|---|
| 1/2 HP | about 600 W | surge about 1,500 – 2,000 W |
| 3/4 HP | about 800 W | surge about 2,000 – 2,500 W |
| 1 HP | about 1,000 W | surge about 2,500 – 3,000 W |
| 1.5 HP | about 1,500 W | surge about 4,000 – 5,000 W |
| 2 HP | about 2,000 W | surge about 5,000 – 6,500 W |
The surge problem
Submersible well pumps use induction motors that pull a large inrush current the instant they start, before the motor is spinning. That surge is typically two to three times the running wattage and lasts only a fraction of a second, but the generator has to deliver it or the pump will not start and may stall the generator.
The practical rule: size to the starting surge. A 1 HP pump that runs at about 1,000 watts can spike near 3,000 watts at start, so a generator that comfortably covers 1,000 running watts but tops out at 2,000 surge watts will not turn it over. This is the single number homeowners most often miss when working out what size generator they need for a well.
The 240-volt requirement
Most residential submersible well pumps from 1/2 HP up are wired for 240 volts. That matters for generator selection because a large share of portable generators rated under about 4 kW only provide 120 volts at their outlets. A 120-volt-only generator cannot run a 240-volt pump no matter how many watts it claims.
So the well pump pushes two requirements at once: enough surge capacity and a true 240-volt output, usually through an L14-30 or similar twist-lock outlet feeding a transfer switch. Confirm both before buying. A licensed electrician installing the transfer switch can confirm the pump voltage and the generator outlet match, and how transfer switch wiring connects shows where the 240-volt circuit lands.
Sizing by pump horsepower
Use the starting surge column to pick the generator, then add the rest of your outage loads on top. The figures below are typical submersible pump numbers; your pump nameplate is the authority.
Soft-start kits and the sump pump pairing
A soft-start kit (a controller that ramps the motor up instead of slamming it on) can cut the starting surge of a well pump substantially, sometimes enough to let a smaller generator run a pump it otherwise could not. For a borderline sizing case, a soft start can be cheaper than the next generator size up, and an electrician can advise whether your pump and controller support one. If you want the well and the rest of the house covered automatically, a whole-house standby generator handles the surge without manual setup.
Plan for the sump pump in the same breath. The storms that knock out power are exactly when a sump pump must keep running, so a well-home backup plan should budget the sump pump running and surge watts alongside the well pump. A 1/3 HP sump pump runs near 800 watts and surges to about 1,300, modest next to the well pump but real when both share the same generator during a wet outage. If either motor trips its breaker on startup, the surge is usually the reason.
Rather talk it through with a pro?
Calls are answered around the clock and routed to a licensed electrical pro serving your area.