What Size Generator Do You Need to Run a Well Pump?

TL;DR

• When the power goes out, a well pump stops and your water stops with it. A generator is how most well owners keep water running.
• Most well pumps run on 240 volts. Your generator needs a 240V outlet (usually labeled L14-30R). A 120V-only generator can't run them.
• Size for the starting surge, not the running load. A pump can pull 2 to 3 times its running watts for a second or two at startup.
• For a typical 1/2 to 1 HP pump plus a few essentials, a generator rated around 7,500 running and 9,500 starting watts covers most homes. Larger pumps need more.
• Connect through a transfer switch or interlock kit installed by an electrician. Never backfeed through an outlet.

On city water, a power outage means no lights. On a well, it also means no water to drink, flush toilets, or wash hands, beyond the few gallons stored in the pressure tank. Winter storms make it worse, since an outage can also freeze the pipes. This guide walks through sizing a generator for your pump, connecting it safely, and what each backup option costs over time.

The Surge-Plus-Everything Rule

A generator has two ratings: running watts (what it can supply continuously) and starting watts (a brief surge it can handle for a few seconds). A well pump's startup surge is what usually decides the size.

Check both numbers:

1. Running watts: add up the running watts of everything you'll power at once (pump, refrigerator, furnace blower, lights). Add 20%. The generator's running watts must meet that.
2. Starting watts: take the running watts of everything except the pump, then add the pump's starting watts. Add 20%. The generator's starting (peak) watts must meet that.

The 20% headroom matters. A generator pushed to its limit drops voltage when the pump kicks on, which can stall the motor, and it wears out faster.

Sizing Chart by Pump Horsepower

Typical figures for 240V submersible pumps, calculated with the rule above. Your pump's nameplate is the real source, and the diagnostic below shows how to find it.

"Essentials" here means a refrigerator, a gas furnace blower, a few lights, and phone chargers, about 2,000 running watts total. Electric heat, an electric water heater, or central air add far more. Starting watts use the middle of each pump's typical range.

Most residential well pumps fall between 1/2 and 1 HP. A generator rated around 7,500 running and 9,500 starting watts, with a 240V outlet, covers any of those pumps plus essentials. That's why it's the most common choice for well owners, and why our pick is the Westinghouse WGen7500DFc. For a 2 HP pump plus essentials, you'll need a larger generator, a soft-start device, or to start the pump before turning anything else on.

5-Minute Diagnostic: Find Your Pump's Specs

You need the pump's horsepower and voltage. Here's where to look:

1. Check the control box. Many submersible pumps have a gray control box mounted near the pressure tank. Its label lists horsepower and voltage. This means you have a 3-wire pump.
2. No control box? You likely have a 2-wire pump, where the controls are built into the pump down in the well. Check your well records or the original installer's paperwork.
3. Check the breaker. A double-pole breaker (two linked switches) for the pump means it runs on 240V. A single breaker usually means 120V, which is rare for submersible pumps.
4. Check the well records. Your well log, home inspection report, or the well contractor's invoice often lists the pump model. Search the model number for its specs.
5. Calculate running watts if you have the amps. Running watts equals volts times amps. A 240V pump drawing 8 amps runs at about 1,920 watts.

Still unsure? A well contractor can tell you on a routine visit. Our guide on how a well pump works explains 2-wire vs. 3-wire systems.

Jet pumps (above-ground pumps for shallow wells) have a nameplate on the motor. Many run on either 115V or 230V. Check which way yours is wired.

Three Things That Change the Math

240V Is Non-Negotiable for Most Pumps

Most well pumps run on 240V. Many smaller portable generators and most inverter generators only put out 120V. Before buying, confirm the generator has a 120/240V outlet, usually an L14-30R twist-lock or a 14-50R. If it doesn't, it can't run a 240V pump, no matter how many watts it's rated for.

Soft Start and Constant-Pressure Pumps Need Less

Some pumps start gently instead of with a big surge:

• Soft-start pumps, such as the Grundfos SQ series, ramp up slowly and need far less starting power.
• Constant-pressure systems with a variable frequency drive (VFD) also ramp up gradually. Check the drive's manual for generator guidance. Some drives need clean power, meaning a generator with low total harmonic distortion (THD), often listed as under 5%.
• Add-on soft-start devices can be installed on some standard pumps to cut the startup surge. Ask a well contractor whether yours is compatible.

If your pump has one of these, you may get by with a smaller generator than the chart shows.

A 30-Amp Connection Caps Your Output

The standard L14-30 connection carries up to about 7,200 watts at 240V. That's enough for most homes running a pump plus essentials. If you need more, you'll need a 50-amp connection (14-50), which costs more to install.

How to Connect a Generator to Your Well Pump

Most well pumps are hardwired to a breaker, so you can't just plug them into a generator. There are three safe ways to connect:

All three need a power inlet box on the outside of the house, a short heavy-duty generator cord, and an electrician. Many areas require a permit.

Never backfeed. A "double male" cord plugged into a dryer outlet or wall outlet can send power back down the utility line, where it can kill a line worker or start a fire. It's illegal in most places and voids insurance coverage. Don't do it.

Generator Safety

Every winter, people die from carbon monoxide poisoning from generators run in the wrong place.

• Outdoors only. Never run a generator in a garage, basement, crawl space, or well house, even with the door open.
• Keep it at least 20 feet from the house, with the exhaust pointed away from doors, windows, and vents.
• Buy a generator with a CO shutoff sensor. Many current models turn themselves off if carbon monoxide builds up around them.
• Put a battery-powered CO alarm inside the house, like the First Alert CO400. Plug-in alarms without a battery backup go silent during an outage, which is exactly when you need them.
• Let it cool before refueling, and store fuel away from the house.

What to Buy

Why dual fuel: propane stores indefinitely and doesn't go stale, which makes it more reliable for a generator that sits unused for months. Gasoline gives a little more power, so it's useful to have both.

Why a CO sensor matters: it's the single most important safety feature on a portable generator. Both picks have one.

The interlock kit isn't on this list because each kit is made for a specific panel brand and model. Your electrician will order the right one.

Before the Next Storm

A generator only helps if it starts. Before winter:

1. Test it monthly. Run it under load for 15 to 20 minutes, ideally powering the well pump through your inlet.
2. Treat or rotate the fuel. Add stabilizer to stored gasoline, or keep a full propane tank on hand.
3. Change the oil on the schedule in the manual. Most new generators need a first oil change after a short break-in period.
4. Keep the cord, a flashlight, and the manual together next to the generator.
5. Fill bathtubs and water jugs when a big storm is forecast, as a backup for flushing and drinking.
6. Know your pressure tank's limit. Most hold only a few gallons of usable water. That's all you'll have until the generator is running.

During a winter outage, freezing is a second risk. See our frozen well pipes guide for what to do if the pump or pipes are exposed to the cold.

Cost Breakdown and 10-Year Total Cost of Ownership

Typical estimates. Fuel costs vary widely with how often and how long your power goes out.

The pattern: a portable generator with a proper connection covers most well owners for a fraction of a standby system. Standby makes sense if outages are frequent, long, or someone at home can't handle a portable generator in a storm.

Common Mistakes

• Buying a 120V-only generator. It won't run a 240V pump, regardless of wattage. Check for an L14-30R or 14-50R outlet.
• Sizing for running watts only. The startup surge is what stalls an undersized generator.
• Backfeeding through an outlet. It's dangerous for utility workers, illegal, and a fire risk. Use an interlock or transfer switch.
• Running the generator in the garage or well house. Carbon monoxide builds up fast, even with doors open.
• Starting everything at once. Start the well pump first, let it settle, then add other loads one at a time.
• Letting fuel go stale. Old gasoline is the top reason generators won't start during a storm. Stabilize it or use propane.
• Never testing it. Find problems on a calm day, not in a blizzard.
• Forgetting the cord. A generator with no way to connect to the house is just a loud box.

When to Call a Pro

• Electrician: installing an interlock kit, transfer switch, or power inlet box. This is required work in most areas, often with a permit.
• Electrician: installing a standby generator and automatic transfer switch.
• Well contractor: confirming your pump's horsepower and voltage if you can't find them.
• Well contractor: adding a soft-start device or upgrading to a soft-start or constant-pressure pump to reduce the generator size you need. See our best well pumps for options.

DIY-friendly: finding your pump specs, buying the generator, and running and maintaining it.