Generator for Well Pump: What Size You Actually Need (2026 Sizing Table)
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How we evaluated: WildJoule has not bench-tested any pump or generator here. Sizing figures come from Franklin Electric’s Submersible Motors: Application, Installation, Maintenance manual (2007 edition, Table 5 and Table 13), Champion’s published wattage chart, University of Nebraska–Lincoln Extension, and pump-maker literature, checked October 10, 2026. This is general guidance; a licensed electrician should handle the connection to your house.
Sizing a generator for well pump backup is where most generic wattage charts let people down. Champion’s chart, for example, lists a 1 HP well pump at 2,000–4,000 starting watts. Franklin Electric, which makes a large share of the submersible motors in American wells, says a 1 HP three-wire motor needs a 4 kW minimum on a typical externally regulated generator, and 50% more than that if the motor is two-wire. That gap is the difference between a pump that starts and a generator that stalls every time the pressure switch clicks.
Short answer
For a common 1 HP, 230 V submersible pump, plan on a generator with a 240 V outlet and at least 4 kW (3-wire pump) or 6 kW (2-wire pump) of rated output, before you add the fridge, furnace and lights. Most homes with a well land in the 7,500–9,500 W class. A 120 V-only inverter generator can’t run a 240 V pump no matter how many watts it has.
I’ll be straight: I don’t have a well, and I’ve never wired a generator to one. What I’ve done is go through Franklin’s motor tables and set them against the generator wattage charts people actually use. When you’ve lived with power that goes out without warning, you learn quickly that losing water is worse than losing lights, which is why this one felt worth getting right.
Generator for well pump: Franklin Electric’s minimum sizes
Franklin’s table gives the minimum generator rating to start and run a submersible motor. It splits generators by how they regulate voltage. Franklin notes that most portable generators are externally regulated, so the left column is the one most buyers should use.
| Pump motor | Externally regulated (most generators) | Internally regulated | 2-wire pump (+50%, external) |
|---|---|---|---|
| 1/3 HP | 1.5 kW | 1.2 kW | 2.25 kW |
| 1/2 HP | 2 kW | 1.5 kW | 3 kW |
| 3/4 HP | 3 kW | 2 kW | 4.5 kW |
| 1 HP | 4 kW | 2.5 kW | 6 kW |
| 1.5 HP | 5 kW | 3 kW | 7.5 kW |
| 2 HP | 7.5 kW | 4 kW | — (Franklin lists 2 HP as 3-wire) |
The first two columns are Franklin’s figures. The 2-wire column is our arithmetic from Franklin’s own rule: “for best starting of 2-wire motors, the minimum generator rating is 50% higher than shown.” Franklin’s table assumes the generator holds at least 65% of rated voltage during the start, a 35% maximum dip. These are minimums for the pump alone. Anything else running on the generator when the pump kicks on has to be added on top.
This table comes from the 2007 edition of Franklin’s AIM manual. Franklin publishes newer editions; if your pump is a Franklin motor, check the current manual for your exact model.
Why generic wattage charts undersize well pumps
A submersible pump motor draws a huge burst of current for the first second, called locked-rotor current. Franklin’s motor data puts a 1/2 HP 230 V two-wire motor at 32.2 locked-rotor amps and a 1 HP 230 V two-wire at 48.7 A, against running power of 960 W and 1,600 W at full service factor. That starting current is several times the running current, and it’s why a generator that looks big enough on running watts bogs down or trips when the pump starts.
Champion’s chart lists a 1/2 HP pump at 1,500–2,000 starting watts and a 1 HP pump at 2,000–4,000. Those ranges sit below what Franklin’s data implies, especially for two-wire motors. When the two disagree, trust the motor maker; the pump is the thing that has to start.
2-wire vs 3-wire: check this before you buy anything
Look for a control box near your pressure tank or in the basement. If there is one, you almost certainly have a 3-wire pump: the box holds the starting capacitor and relay. If there’s no box and the wires run straight from the pressure switch down the well, it’s probably a 2-wire pump, with the starting components built into the motor.
Two-wire motors draw more starting current. In Franklin’s data a 1/2 HP 230 V two-wire motor pulls 32.2 locked-rotor amps versus 23 A for the three-wire version. That’s the reason for the 50% bump. Two-wire motors also don’t like being restarted within about five seconds of losing power, which matters when you switch from grid to generator: wait a moment before the pump tries to start again.
Voltage: why most well pumps need a 240V generator
In Franklin’s single-phase motor table, 115 V is only offered at 1/2 HP. Every submersible from 3/4 HP up is 230 V. Jet pumps (common on shallow wells) are often dual-voltage, and up to about 1.5 HP can be wired for 115 V according to Connecticut’s private-well training material, but plenty are wired for 230 V anyway.
That rules out most small inverter generators. A Honda EU2200i, Yamaha EF2200iS or Champion 4,500 W inverter is 120 V only. They can’t power a 240 V pump at any load. You need a generator with a 120/240 V outlet, usually an L14-30R or 14-50R, connected through transfer equipment. Our split phase inverter guide explains the 120/240 V setup if you’re going the battery route instead.
Sizing a generator for well pump plus the rest of the house
A worked example, using a 1 HP 230 V two-wire submersible and a typical essentials list:
- Pump minimum from Franklin (2-wire, externally regulated): 6 kW
- Fridge, freezer, furnace blower, lights and internet already running when the pump starts: add their running watts on top (check each label; our portable generator buying guide lists typical figures)
- Leave headroom: generators sag when run near their limit, and dual-fuel units make less on propane
That’s why most well households end up with something like a 7,500–9,500 W dual-fuel open-frame generator. The Westinghouse WGen9500DF, for example, is rated 9,500 W running on gasoline and 8,500 W on propane, with an L14-30R and a 14-50R. That’s enough for a 1 HP two-wire pump plus essentials with room to spare. Our best dual fuel generator for home backup compares it with the alternatives.
Soft starters and constant-pressure drives
If the generator for well pump duty you already own is borderline, a soft starter can help. SoftStart claims its well-pump starter cuts inrush current by up to 40%. That’s the maker’s claim, without a stated baseline, so treat it as a best case. Variable-speed drives like Franklin’s SubDrive ramp the motor up instead of hitting it with full voltage; Franklin’s sizing rule for those systems is 1.5 times the drive’s maximum input watts. Grundfos SQ pumps have a soft start built in.
A soft starter fixes the surge, not the voltage. It won’t make a 120 V generator run a 240 V pump.
Connecting it safely
Franklin’s manual says transfer switches must be used any time a generator backs up a building’s wiring, and the National Electrical Code requires transfer equipment that prevents the generator and the grid from being connected at the same time. A breaker interlock only counts if it’s listed for your specific panel. Never backfeed through a dryer outlet with a double-male cord. Our generator transfer switch guide compares manual switches, interlocks and automatic switches, and the generator extension cord guide covers the L14-30 cord to the inlet.
Franklin adds two habits worth keeping: start the generator before the pump, and turn the pump off before shutting the generator down. Letting a generator coast down with the motor still running can damage the motor’s thrust bearing.
Your pressure tank buys you time
The pressure tank stores water so the pump starts less often. University of Nebraska–Lincoln Extension describes typical settings where the pump cuts in at 20–40 psi and off at 40–60 psi. Fewer starts means fewer surges on the generator. Franklin also limits starts to protect the motor: up to 300 per day for pumps to 3/4 HP and 100 per day for 1 HP and larger, with at least one minute of running each time. A waterlogged or undersized tank that makes the pump short-cycle is hard on both the motor and the generator.
If the pump is only one of several loads, add the rest with our generator size chart. A 240 V well pump also needs a 240 V path into the house, which means a transfer switch or interlock rather than a cord through a window.
Frequently asked questions
What size generator do I need for a 1 HP well pump?
Franklin Electric’s minimum is 4 kW for a 3-wire motor on a typical externally regulated generator, and 50% more (6 kW) for a 2-wire motor. It must have 240 V output if the pump is 230 V, and you add your other loads on top.
Can an inverter generator run a well pump?
Only a 120/240 V one that’s large enough. Most portable inverter generators, including the 2,200 W and 4,500 W classes, are 120 V only and can’t run a 230 V pump.
How do I know if my well pump is 2-wire or 3-wire?
A separate control box near the pressure tank usually means a 3-wire pump. No box usually means 2-wire. The motor nameplate or install paperwork will confirm it.
Will a soft starter let me use a smaller generator?
It can reduce the starting surge (SoftStart claims up to 40%), which may let a borderline generator cope. It doesn’t change the voltage requirement.
Further reading: University of Nebraska–Lincoln Extension’s guide to private water well systems, and the main portable generator buying guide for everything else on the outage list.
The same starting-surge logic applies to the other loads on an outage list. See our guides to sizing a generator for sump pump backup and a generator for furnace blowers, and how long will a generator run once you’ve picked one.
Last updated: October 10, 2026. This generator for well pump guide uses Franklin Electric’s published motor and generator data; confirm against your own pump’s nameplate. We do not display live prices.