Home / Generators

Generator for a well pump: size it from the motor, not a guess

By the Plain Prepper editors. Updated 2026-10-15.

We research products from makers' specs, manuals and safety agencies; we do not run hands-on tests. Links to Amazon may earn us a commission at no cost to you. How this works.

Quick answer

Start with the pump motor's horsepower and whether it is a 2-wire or 3-wire motor. Franklin Electric, which makes submersible well motors, lists a minimum generator rating of 4 kW for a 1 HP 3-wire motor and 6 kW for a 1 HP 2-wire motor. If your pump runs on 240 V, the generator needs a 120/240 V outlet. Franklin also says a transfer switch must be used whenever a generator backs up a home's power lines; have a licensed electrician install it.

Franklin Electric's minimum generator sizes

Franklin Electric's application manual lists the smallest generator it considers adequate for its single-phase submersible motors:

Motor	3-wire motor	2-wire motor
1/2 HP	2 kW	3 kW
3/4 HP	3 kW	4.5 kW
1 HP	4 kW	6 kW
1.5 HP	5 kW	7.5 kW
2 HP	7.5 kW	Not listed
3 HP	10 kW	Not listed

These are minimums for the pump alone. Anything else you run at the same time, such as a refrigerator or lights, adds to the total. Franklin's reason for the margin: a generator must hold at least 65% of the motor's nameplate voltage while it starts, or the motor may not get enough starting torque.

Find your pump's numbers first

• Horsepower: on the motor nameplate or the pump paperwork.
• 2-wire or 3-wire: Franklin lists them separately because 2-wire motors need more generator. Ask your well contractor if you are unsure which you have.
• Voltage: check whether the pump runs on 120 V or 240 V before buying a generator.
• Elevation and fuel: Franklin says to follow the generator maker's derating advice at high elevation or when running on natural gas.

Two generators compared against the table

Spec	Champion 201277	Champion 201302
Running watts, gasoline	7,500	9,200
Running watts, propane	6,750	8,280
Starting watts, gasoline	9,375	11,500
240 V outlets	L14-30R	L14-30R and 14-50R
Weight	205 lb	209 lb

Against Franklin's minimums, both cover a 1 HP 2-wire pump (6 kW) on either fuel. A 1.5 HP 2-wire or 2 HP 3-wire pump (7.5 kW) uses the 201277's entire gasoline running rating with nothing left over, while the 201302 leaves about 1,700 W of running headroom on gasoline.

Connecting it: transfer switch, not a backfeed

Franklin Electric says automatic or manual transfer switches must be used any time a generator is used as standby or backup on power lines, to prevent accidental electrocution, and to contact the power company for use and approval. NFPA also calls for a properly rated transfer switch.

• A 30 amp manual transfer switch kit pairs with a generator's L14-30R outlet. Connecticut Electric's manual says its EmerGen switch must be installed by a licensed electrician or qualified professional under local code.
• If your pump is hard-wired, there is no plug to connect, so plan the connection with an electrician.

See our transfer switch for portable generator guide for switch types.

Running the pump on a generator

1. Start the generator first, then turn on the pump circuit. Franklin says to always start the generator before the motor.
2. Shut the pump off first, then the generator. Franklin warns the motor's thrust bearing can be damaged if the generator coasts down with the motor running or runs out of fuel.
3. Keep fuel up so the generator never dies under the pump's load.
4. No reduced voltage starter with a minimum-sized generator; Franklin says the combination can cause motor damage from low voltage.
5. Frequency matters: Franklin notes a pump running 1 to 2 Hz below its design will not meet its performance curve, and 1 to 2 Hz above may trip overloads.

NFPA says to run portable generators outdoors, at least 20 feet from windows, doors and vent openings.

What to look for when you buy

Questions people ask

What size generator do I need for a 1 HP well pump?

Franklin Electric lists a minimum of 4 kW for a 1 HP 3-wire motor and 6 kW for a 1 HP 2-wire motor, before other loads.

Can I plug a well pump into a generator?

Not safely by improvising; Franklin says a transfer switch must be used when a generator backs up power lines, installed by an electrician.

Will a 7,500 watt generator run a well pump?

For most pumps up to 1 HP 2-wire or 2 HP 3-wire, it meets Franklin's minimums, though the larger sizes leave no margin for other loads.

Why does my pump need such a large generator?

Franklin says the generator must hold at least 65% of the motor's nameplate voltage while it starts, so it needs capacity well above running watts.

Sources

• Application, Installation, Maintenance (AIM) Manual, 60 Hz, Franklin Electric
• 201277 7500W Dual Fuel Generator Operator's Manual, Champion Power Equipment
• 201302 9200W Electric Start Dual Fuel Generator, Champion Power Equipment
• EmerGen Switch G2 installation manual, Connecticut Electric
• Generator safety tips, National Fire Protection Association

Sources checked 2026-10-15.