Generator Size for a Well Pump
The right well pump generator is one that can handle the pump's starting surge, not just its running load. Angi's rule of thumb is that starting wattage is typically two to three times the running wattage, so size the generator for that surge plus everything else that will be running when the pump kicks on.
You get the pump's numbers from its nameplate or manual, not from a chart. Pumps of the same horsepower can draw quite differently.
How to size it, step by step
- Find the pump's volts and amps. Submersible pumps have a nameplate on the control box in the basement, or the data is in the install paperwork. Jet pumps have a label on the motor.
- Work out running watts. When the label lists amps but not watts, multiply volts by amps.
- Estimate the starting surge. Plan on two to three times the running figure. If the pump maker publishes a generator sizing table, use that instead. It beats any rule of thumb.
- Add what else runs at the same time. Fridge, freezer, furnace fan, sump pump, lights. Add their running watts.
- Check that the generator's surge rating covers the pump start on top of those running loads. Honda's advice is to stagger high-start appliances rather than start them together, which lets a smaller generator get by.
Then check voltage. If the pump runs on the higher household voltage (the same supply as an electric dryer or range), the generator needs a full-power outlet at that voltage, not just its standard household receptacles.
Why well pumps are hard on generators
A well pump is one of the biggest motor loads in a house, and it starts against a full column of water. The surge lasts only a moment, but if the generator can't cover it, the voltage sags, the pump struggles, and the motor runs hot. Do that enough times and you shorten the motor's life.
So the useful number is the generator's surge (starting) rating, read alongside its running rating. A generator that's fine on paper for running watts can still stall every time the pressure switch closes.
Constant-pressure pumps with a variable-speed drive ramp the motor up gently, which cuts that surge. If you're replacing a pump anyway, it's worth asking about. See constant pressure well pumps.
Numbers that matter
| What to check | Why |
|---|---|
| Pump running watts (volts × amps) | The steady load |
| Pump starting surge | Usually two to three times running |
| Other loads during a pump start | Sump pumps alone are listed at 800 to 1,050 watts by Honda |
| Generator outlet rating | Has to match the transfer switch inlet and cord, such as Generac's 30- or 50-amp HomeLink kits |
Angi's circuit guidance helps too. It pairs generators up to 5,000 watts with transfer switches of 6 to 10 circuits, and 5,000 to 10,000 watt generators with 10 to 16. Your well pump takes one of those circuits, usually a two-pole one.
The pressure tank buys you a little slack. It holds some water under pressure, so the pump doesn't have to run every time someone opens a tap. It doesn't change the surge the generator has to cover once the pump does start.
Connecting the generator safely
A well pump is hard-wired, so a cord won't reach it. You need one of these:
- A manual transfer switch. Generac's HomeLink switches are made to let a portable generator run hard-wired loads like a well pump. Installed, manual switches cost $400 to $1,300.
- A panel interlock kit. A sliding plate locks out the main breaker so a back-fed generator breaker can feed the panel. HomeGuide puts installed interlocks at $400 to $850.
- A standby generator with an automatic transfer switch. The pump keeps running without anyone going outside. See automatic transfer switches.
For the switch side of this, see the manual transfer switch guide or interlock kits.
Battery and solar backup for a well pump
Battery systems can run a well pump, but the same surge rule applies to the inverter. The battery's surge rating has to cover the pump start, and the pump's runtime eats capacity fast. Check the battery maker's motor-load guidance before you count on it. See home batteries.
Solar well pump kits with their own battery are a different setup, usually a dedicated low-voltage pump. They make sense for off-grid wells and livestock more than as backup for a house pump. See solar well pumps. For a no-power fallback, some owners add a hand pump.
FAQ
What size generator do I need to run a well pump?
Read the pump's volts and amps, multiply them for running watts, and plan on two to three times that for the start. Add the running watts of anything else that'll be on, and pick a generator whose surge rating covers the total. If the pump's manual has a generator table, follow it.
How do I reset a well pump pressure switch after a power outage?
Most switches reset themselves when power returns. If yours has a side lever, it's a low-pressure cutoff switch, which can lock the pump off when pressure falls too far. With the pump breaker on, hold the lever in the start position until the gauge climbs past the cutoff, then let go. See the low-pressure cutoff switch guide for the full steps.
Can a portable generator run a well pump?
Yes, if its surge rating covers the pump start and it has an outlet at the pump's voltage. It also needs a transfer switch or interlock, since a well pump is hard-wired. Start the pump with other big loads off, as Honda advises for high-start appliances.
Why does my generator stall when the well pump starts?
The pump's starting surge is more than the generator can deliver with everything else running. Turn off other loads and try again. If it still stalls, the generator is undersized for that pump.
Sources
- AngiWhat Size Generator Do I Need? (opens in a new tab)
- Honda Power EquipmentHow Much Power Do You Need? (opens in a new tab)
- GeneracHomeLink Transfer Switches (opens in a new tab)
- AngiHow Much Does It Cost to Install a Generator Transfer Switch? (opens in a new tab)
- HomeGuideGenerator Transfer Switch Installation Cost (opens in a new tab)
- Honda Power EquipmentGenerator Wattage Calculator (opens in a new tab)