System guide
What Size Generator Runs a Well Pump?
When the power goes out, the well goes with it. The generator that brings it back has to be sized for the pump's start, not its run — and connected the right way.
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Franklin Electric's minimum generator sizes for its three-wire submersible motors are 2 kW for ½ HP, 3 kW for ¾ HP, 4 kW for 1 HP and 5 kW for 1½ HP. A two-wire motor needs a generator 50% larger.[1] In Franklin's line, motors of ¾ HP and up are 230-volt, so the generator usually needs a 240-volt outlet as well. Connect it through a transfer switch, and run it outdoors at least 20 feet from the house.[2]
Two ways generators kill
- Carbon monoxide. The U.S. Consumer Product Safety Commission says to operate portable generators outside only, at least 20 feet from the house, and never inside a home, garage, basement, crawlspace or shed — nor on a porch or in a carport. It attributes about 100 deaths a year to portable-generator carbon monoxide.[2]
- Electrocution. Franklin's manual warns that, to prevent accidental electrocution, an automatic or manual transfer switch must be used any time a generator is used as standby or backup on power lines, and says to contact the power company for use and approval.[1] Never feed a generator into the house through an outlet.
Minimum generator size by pump horsepower
| Pump motor | 3-wire motor | 2-wire motor (+50%) |
|---|---|---|
| ½ HP | 2 kW | 3 kW |
| ¾ HP | 3 kW | 4.5 kW |
| 1 HP | 4 kW | 6 kW |
| 1½ HP | 5 kW | 7.5 kW |
| 2 HP | 7.5 kW | — |
| 3 HP | 10 kW | — |
| 5 HP | 15 kW | — |
The three-wire column is Franklin's Table 5 for externally regulated generators, which it describes as the most common kind.[1] The two-wire column is our arithmetic from Franklin's note that for best starting of two-wire motors "the minimum generator rating is 50% higher than shown." Franklin's 4-inch two-wire line stops at 1½ HP, which is why the last three rows are blank.
How to read the table
- These are minimums for the pump alone. Anything else running on the generator adds to the requirement.
- Franklin bases the table on continuous-duty generators, so compare it with a generator's rated (running) watts — not the larger peak or starting number on the box. That comparison is our reading of the table, not Franklin's wording.
- Franklin calls it "a general chart" and says the generator manufacturer should be consulted whenever possible.
- Other motor brands publish their own figures. Use your pump's documentation where you have it.
Not sure which kind of motor you have? A control box on the wall near the pressure tank means three-wire. See 2-wire vs. 3-wire well pumps.
Why the generator has to be so much bigger than the pump
A 1 HP motor uses about 1.2 kW once it is running. The table asks for 4 kW or more because of the instant it starts. Franklin lists the same 1 HP, 230-volt three-wire motor at 8.2 amps running at full load and 41.8 amps with the rotor locked, which is what the motor draws at the moment of starting — about five times as much. The two-wire version draws 48.7 amps.[1]
A generator that is too small can't hold its voltage through that surge. Franklin's rule is that the generator must deliver at least 65% of rated voltage during starting to give the motor enough starting torque.[1] If it can't, the motor may fail to start properly — and the same manual warns that low voltage during starting carries the potential for severe motor damage.
Check the voltage before you buy
- Many submersibles are 230-volt. In Franklin's 4-inch single-phase line, ½ HP motors come in 115-volt and 230-volt versions; ¾ HP and larger are 230-volt only.[1]
- How to tell: the pump's breaker is the quickest clue. A double-pole breaker (two handles tied together) feeds a 240-volt circuit. The motor's paperwork or the control box label states the voltage outright.
- The generator needs a matching outlet. A 230-volt pump needs a generator with a 240-volt output. Smaller portables often provide 120 volts only, and no adapter fixes that.
Connecting it: transfer switch, not extension cord
A well pump is hard-wired; there is no plug to move to the generator. The safe arrangement is a transfer switch or interlock installed by a licensed electrician, which disconnects the house (or the pump circuit) from the utility before the generator is connected. That is what stops generator power from flowing back out onto lines a utility crew may be working on, and it is the arrangement Franklin's electrocution warning requires.[1]
Professional-only
Installing a transfer switch, an inlet box or a pump disconnect is electrician's work, and usually needs a permit and the utility's approval. Do not wire a generator to the pressure switch or control box yourself.
Running the pump on a generator
Franklin gives four operating rules:[1]
- Start the generator first. Let it come up to speed, then switch the pump on.
- Stop the pump before the generator. Switch the pump's breaker off before shutting the generator down.
- Don't let it run out of fuel with the pump on. Franklin warns that the motor's thrust bearing may be damaged if the generator coasts down with the motor running, and that running out of fuel causes the same condition.
- Watch the frequency. A pump running 1 to 2 Hz below its 60 Hz rating won't meet its performance curve; 1 to 2 Hz above may trip the overloads.
Franklin also says to follow the generator maker's recommendations for derating at higher elevations or when running on natural gas — a generator delivers less than its nameplate in both cases.[1]
If you have a constant-pressure drive
Variable-speed systems are sized differently. Franklin's rule for its SubDrive and MonoDrive controllers is 1.5 times the drive's maximum input watts, rounded up to the next standard generator size:[1]
| Controller | Minimum generator |
|---|---|
| MonoDrive, ½ HP | 2,000 W |
| MonoDrive, ¾ HP | 3,000 W |
| MonoDrive, 1 HP | 3,500 W |
| MonoDriveXT, 1½ HP | 4,000 W |
| MonoDriveXT, 2 HP | 5,000 W |
| SubDrive15 / SubDrive75 | 3,500 W |
| SubDrive20 / SubDrive100 | 5,700 W (6 kW) |
| SubDrive30 / SubDrive150 | 7,000 W |
| SubDrive2W | 6,000 W |
Franklin adds that these drives are not to be used on a ground-fault circuit interrupter (GFCI), and that the generator's voltage and frequency must be appropriate for the drive.[1] Other brands have their own requirements; check the drive's manual. More on these systems in constant-pressure well systems.
How much water you have with no power
Only what is already in the pressure tank — and that is much less than the number on its label. By the calculation method published in The Driller, a 40-gallon tank on a 40/60 switch holds about 10 gallons of usable water between pump starts, and less if the tank was already partly drawn down when the power failed.[3] See real drawdown by tank size. For an expected outage, filling containers beforehand is the cheapest backup there is.
Sources
- Franklin Electric. Submersible Motors: Application, Installation & Maintenance (AIM) Manual, 60 Hz. Accessed Sept. 28, 2026.
- U.S. Consumer Product Safety Commission. As Winter Storms Threaten Millions in the U.S., CPSC Issues Safety Tips to Help Families Prevent Carbon Monoxide Poisoning and Fires (Dec. 12, 2025). Accessed Oct. 8, 2026.
- The Driller. How to Calculate and Control Pressure Tank Drawdown — Bob Pelikan (Feb. 1, 2006). Accessed Sept. 28, 2026.
We cite manufacturer documentation and public agencies first. Read how we choose and check sources.