Well pump on backup power

The lights come back on the generator. The fridge hums. Then someone flushes a toilet, the pressure tank is empty, the well tries to start, and the generator dies or the pump just clicks. You still do not have water.

A well pump is a motor at the bottom of a pipe, starting against a column of water. The quiet running watts on a sales chart are not the problem. The locked-rotor amps (LRA) on the motor nameplate are. If that circuit is not on the critical-load panel, the rest of the backup plan is a dark house with a working modem.

This is electrician work. A 240-volt submersible is not an extension-cord job, and a cord in the dryer outlet is still how people energize lines a lineman thinks are dead. The inlet box and interlock vs transfer switch pages are the legal hookup.

What does the nameplate actually say?

Open the control box by the pressure tank, or the well file if you still have it. You want voltage, horsepower, full-load amps, and LRA. Two-wire motors hide the start parts down the hole. Three-wire motors have a control box on the wall. Most residential submersibles are 230 volts. A 120-volt inverter in the garage will not run that motor. It will try, and you can wreck the windings.

Franklin Electric’s AIM tables for 4-inch, 230-volt, 60 Hz, 2-wire motors (the common residential class) list full-load watts and locked-rotor amps like this:

• 1/2 horsepower: about 670 watts running, 32.2 LRA
• 3/4 horsepower: about 940 watts running, 40.7 LRA
• 1 horsepower: about 1,210 watts running, 48.7 LRA
• 1.5 horsepower: about 1,770 watts running, 66.2 LRA

Franklin Electric AIM motor tables (PDF)

Starting watts are LRA times volts. On 230 volts, that 1 horsepower motor wants on the order of 11,200 watts for the first moment, then settles near 1,200. A 3-wire 1 horsepower motor in the same book is a little kinder (about 41.8 LRA, still around 9,600 watts to break loose). Generator blogs that say “1 HP, 6,000 starting watts” are a different, softer chart. Believe the plate on your motor.

If the plate is painted over, the well contractor who last pulled the pump can often tell you the model. Guessing horsepower from the house square footage is how people buy a 7,500-watt portable that never starts the well.

A 1/2 horsepower motor on 115 volts in that same Franklin table still wants about 670 watts running, but locked-rotor is 64.4 amps. The watt spike is similar. The generator problem is different: you need a 120-volt outlet that can feed that inrush, and you still cannot use that machine on a 230-volt drop. Jet pumps in a shallow well are often this 120-volt class. Submersibles past about 25 feet of lift are usually 230. Do not assume.

Why do the generator charts look so small?

HomeGuide’s 2026 generator cost page lists a 7 to 10 kW portable as covering lights, a fridge, a sump, a well pump, and a furnace fan. That coverage line is a running-load story. It is not an LRA test against a 48-amp locked rotor.

HomeGuide: generator cost

A 30-amp, 240-volt inlet (the common L14-30 hookup) is 7,200 watts on paper: 30 amps times 240 volts. The inlet page is the hardware. If the pump wants 40 to 50 amps for a second, that 30-amp generator breaker opens, or the voltage sags so far the motor never gets spinning. Repeated failed starts cook a submersible that you cannot see.

A 50-amp inlet (14-50, about 12,000 watts theoretical) is the next portable class, and it still has to match a generator that can actually deliver that surge, not a sticker that says 12,000 starting watts into a resistive heater.

Motors do not all start at once. Size for running watts of everything you will leave on, plus the single worst start. In a well house that worst start is usually the well, unless you also have central air on the same backup. If the well and the air conditioner can start together, you are sizing for both, which is how a modest ranch gets quoted a 22 kW standby.

What are the ways to make the start smaller?

A bigger pressure tank. The pump runs less often. It does not lower LRA. It just means fewer punches during a long outage.

A soft starter or a constant-pressure drive. These ramp the motor instead of slamming full voltage. They are a well-contractor and electrician job, matched to that motor. They can be the difference between a portable that works and a portable that trips. They are not a $20 Amazon adapter.

Start the generator first, then close the well breaker. If other motors are already running, give the well the surge budget by itself. Turn the well off before you shut the generator down, so you do not stall the engine with the pump still loaded.

Do not run the electric water heater and the well as if they were lights. A 4,500-watt heater is a running load with no surge, and it will eat the headroom the pump needs to start.

A battery inverter has the same two tests: 240 volts, and surge. A 120-volt power station that can run a fridge will not run a 230-volt submersible. Pairing two 120-volt boxes to fake 240 is a manufacturer-specific kit, not a hope.

How does the well get onto the backup?

Trace the well breaker. It is usually a two-pole 240-volt breaker. On a six-to-ten circuit manual switch, that breaker is two slots, and it is the one people skip because they wired the kitchen first. If the well lives in a yard panel or a barn, the house transfer switch never sees it. The first outage is how you find out.

Ask the electrician, in writing:

1. Is the well pump circuit on this transfer equipment, including a well that lands in a subpanel?
2. What voltage and LRA did they use to size the generator or battery?
3. 30-amp or 50-amp inlet, and does that match the generator outlet and the cord?
4. Interlock, manual switch, or automatic, and is the kit listed for this panel? That fight is on the interlock page.
5. Soft starter or drive, if the portable you already own cannot make LRA.

Standby with an automatic transfer switch starts without you. That matters if you are away, if the well also feeds a livestock tank, or if a sump and a well both need to catch up when you get home. The automatic vs manual page is that choice. The sizing page is the rest of the house.

What does this cost?

HomeGuide’s 2026 generator page puts portables at $500 to $2,500 for the machine, or $900 to $4,000 if you connect it to the house. A whole-house standby is $6,000 to $11,000 installed in their typical band, and $9,500 to $24,000 when the run is long or the unit is over 20 kW. Automatic transfer switch installation in that writeup is $600 to $2,500. Their interlock-plus-inlet band, reused on our inlet page, is $400 to $850.

Those dollars do not include a new well pump. If the motor is dying, replacing it with a constant-pressure unit that starts softer can be cheaper than jumping from a 10 kW portable to a 22 kW standby. That is a well quote, not a generator quote.

The battery vs standby comparison still holds: a battery that cannot make 240 volts and the LRA spike is a fridge battery. A standby that never got the well circuit moved is a lights generator.

A worked pair of bids

Same house: 1 horsepower, 230-volt, 3-wire submersible, Franklin-class nameplate around 8 amps running and the mid-40s LRA. Fridge, furnace blower, lights, internet. No central air on backup.

Bid A. 7,500-watt portable, 30-amp inlet, interlock, well not listed on the circuit map. HomeGuide portable-plus-hookup money, $900 to $4,000. On paper it runs the fridge. When the tank empties, the 30-amp breaker and the ~11 kW start do not get along. You have backup without water.

Bid B. Same portable class, but the electrician puts the well on the switch, measures or reads LRA, and either steps up to a 50-amp generator that can actually surge or adds a soft start the well contractor will warrant. Or they skip the portable fight and quote a 13 to 16 kW natural-gas standby with the well on the automatic switch. HomeGuide’s 13 to 16 kW natural-gas installed band is $6,000 to $10,000.

HomeGuide: natural gas generator cost

If Bid B is twice Bid A, check whether you are buying water or buying the rest of the house. If Bid A never mentions LRA or 240 volts, it is not a well-pump bid.

What goes wrong after you sign?

The well is on a yard panel. The transfer switch is in the basement. Nobody tied them together.

The generator is 120-volt only. The pump is 230.

The sales chart said 6,000 starting watts. The plate says 48 LRA.

Someone runs the pump from a cord in the dryer outlet.

The soft starter is a generic VFD that the motor manufacturer will not support down a 200-foot drop.

The pressure tank is waterlogged, so the pump short-cycles all night on a generator that was sized for one start an hour.

How do I decide this week?

1. Read the pump nameplate or control box for volts, horsepower, full-load amps, and locked-rotor amps, and write those numbers on the generator quote.
2. Trace the well breaker and ask whether that two-pole circuit is actually on the inlet, interlock, or transfer switch, including a well that lands in another panel.
3. Multiply LRA by voltage and compare that start to the generator’s 30-amp or 50-amp outlet, not to a blog chart for a generic 1 horsepower pump.
4. Decide whether you will be home to start a portable, or whether the well has to come back without you.
5. Price a soft start or a constant-pressure replacement against jumping a whole generator class, using the well contractor for the motor and the electrician for the hookup.
6. Keep the generator outside, and keep the well off until the engine is up to speed.

You can keep well water during an outage. You cannot tell that from a 7,500-watt sticker that never saw your LRA.