How to Run Your Well Pump on a Generator During a Power Outage
No power means no water on a well. Here's how to size a generator for a submersible pump, hook it up safely and start it in the right order.
A well pump stops when the power goes out, and so does your water. To run it on a generator, you need three things. First, a generator big enough for the pump's starting surge: Franklin Electric lists a 4 kW minimum for a 1 HP, 3-wire motor. Second, 240-volt output. Third, a transfer switch or interlock so the generator can't feed power back into the utility lines.
Outages here can last a long time. During Winter Storm Uri in February 2021, close to 4.5 million Texas homes and businesses lost power at the peak. A University of Houston survey found that 69% of Texans lost power at some point between February 14 and 20, for an average of 42 hours.
First, learn your pump's size and wiring
You need two facts before you shop: the motor's horsepower and whether it's a 2-wire or 3-wire motor.
- Look for a control box. A 3-wire submersible motor uses a separate control box, a small box mounted near the pressure switch or in the well house, with a label that lists the horsepower and voltage. If the wires run from the pressure switch straight to the well with no control box, it's most likely a 2-wire motor.
- Check your paperwork. The invoice from the last pump installation usually lists the horsepower. Your State Well Report shows the pump type and how deep it was set, but not its horsepower. Here's how to find and read that report.
- Constant-pressure systems are different. If your pump runs on a variable-speed drive (a "constant pressure" system), follow the generator sizing in that drive's manual instead of the table below.
Voltage matters too. Franklin Electric's 4-inch single-phase submersible motors come in 115 volts only at 1/2 HP. From 3/4 HP up they're 230-volt motors. Most well pumps can't run from a regular 120-volt outlet on a generator.
What size generator a well pump needs
These are Franklin Electric's minimum generator ratings for its submersible motors, from its 2026 Application, Installation and Maintenance (AIM) manual:
| Motor size | 2-wire motor: minimum generator | 3-wire motor: minimum generator |
|---|---|---|
| 1/2 HP | 3 kW | 2 kW |
| 3/4 HP | 4.5 kW | 3 kW |
| 1 HP | 6 kW | 4 kW |
| 1 1/2 HP | 7.5 kW | 5 kW |
| 2 HP | — | 7.5 kW |
| 3 HP | — | 10 kW |
| 5 HP | — | 15 kW |
Three things to keep in mind:
- These figures cover the pump alone. Add whatever else you plan to run at the same time, such as the refrigerator, freezer, lights and fans.
- Starting is the hard part. Franklin Electric says a generator must deliver at least 65% of the motor's nameplate voltage during starting so the motor can get up to speed.
- Derate when needed. Franklin Electric says to follow the generator maker's derating guidance for higher elevations or natural gas fuel.
If your pump is from another manufacturer, check that maker's generator sizing chart. The table above is a reasonable starting point for comparison, not a substitute.
Connect it the safe way
Use a transfer switch or interlock. Franklin Electric's manual says "automatic or manual transfer switches must be used any time a generator is used as standby or back up on power lines," and to contact the power company for approval. A licensed electrician can install a generator inlet and a transfer switch or interlock at your panel, so the well circuit runs on generator power with the utility side cut off.
Never backfeed. The U.S. Consumer Product Safety Commission warns: "NEVER try to power the house wiring by plugging the generator into a wall outlet, a practice known as 'backfeeding.'" It puts utility workers and neighbors on the same transformer at risk of electrocution.
Keep the generator outside. The CPSC says to run portable generators "outside only, at least 20 feet away from the house," with the exhaust pointed away from the home. Never run one in a garage, shed, crawlspace or on a porch, and keep battery-powered carbon monoxide alarms working inside.
Start and stop in the right order
Franklin Electric gives clear rules for running a submersible on a generator:
- Start the generator first, let it come up to speed, and then switch on the well circuit.
- Turn the pump off before you shut the generator down.
- Don't let the generator run out of fuel with the pump running. Franklin Electric warns that the motor's thrust bearing can be damaged if the generator coasts down with the motor running.
Starts matter as well. Franklin Electric recommends that a single-phase motor of 1 to 5.5 HP start no more than 100 times in 24 hours, and that each run last at least one minute. If you're saving fuel by running the generator in blocks, use those blocks for the heavy jobs: refilling the pressure tank, filling containers, showers and laundry.
Plan before the next outage
- Store water anyway. Ready.gov recommends one gallon per person per day for several days, for drinking and sanitation. The pressure tank holds only a few gallons of usable water once the pump stops.
- Fill a bathtub before a forecast storm for flushing toilets.
- Test-run the generator before storm season, under load, with fresh fuel.
- Label the well breaker at the panel and on the transfer switch.
If a storm also flooded your well house, don't run the pump until it has been checked. See what to do with your well after a flood.
When the power comes back
Switch the transfer switch back to utility power, then make sure the pump starts and the pressure climbs normally. If the breaker holds but the pump stays silent, or the pressure won't build, a power surge may have damaged the pressure switch, the control box or the motor. In the Weatherford area, Water Well Drilling Weatherford takes no-water calls 24/7, including nights and weekends.
Quick answers
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 submersible motor and 6 kW for a 1 HP, 2-wire motor. Add the other loads you plan to run at the same time.
Can I plug my well pump into a portable generator? Most submersible well pumps are wired straight to a 240-volt circuit, not plugged in. The safe connection is a generator inlet with a transfer switch or interlock, installed by a licensed electrician. Never backfeed through a wall or dryer outlet.
Will a 3,500-watt generator run a well pump? Only a small one. A generator rated for 3,500 running watts meets Franklin Electric's minimum for a 3-wire motor up to 3/4 HP or a 1/2 HP 2-wire motor. It falls short of the 4.5 kW minimum for a 3/4 HP 2-wire motor and the 4 kW minimum for a 1 HP 3-wire motor, before you add any other loads.
