Sizing a generator for a well pump

The USGS estimates that more than 43 million people in the United States get drinking water from private wells. When the power goes out, the pump stops, and so do the toilets, showers, and kitchen tap. For well owners, the pump is often the load that decides generator size.

Typical well pump wattage

Champion Power Equipment lists:

Pump	Running watts	Starting watts
1/2 HP well pump	750–1,000	1,500–2,000
1 HP well pump	1,000–2,000	2,000–4,000

Larger pumps (1.5 to 2 HP and up) draw more. Deep submersible pumps often have high starting current, and the surge is the number that stalls undersized generators.

120 V or 240 V?

Many deep-well submersible pumps run on 240 volts. That matters for two reasons. First, the generator must produce 240 V, which usually means a conventional generator of about 5,000 watts or more, or a large inverter model with 240 V output. Second, the pump is hard-wired, so it must be connected through a transfer switch or interlock, not an extension cord.

Find your pump’s real numbers

• Look at the pressure switch or control box (often in the basement or well house). The control box label lists horsepower, volts, and amps.
• The pump’s breaker size gives a clue; a two-pole breaker means 240 V.
• Your well driller’s records usually list pump model and horsepower.

Multiply the running amps by volts for running watts. If the label lists “service factor amps,” use that as a conservative figure.

Sizing example

A 1 HP 240 V submersible (2,000 running, 4,000 starting from the chart) plus a refrigerator (400), furnace fan (800), and lights and internet (250):

• Running total: 3,450 W
• Largest extra surge: the well pump, 2,000 W
• Peak: 5,450 W
• With 20% headroom on running: about 4,300 running watts

A generator of roughly 5,000 to 7,500 running watts with 240 V output and a starting rating well above 5,450 W gives comfortable margin. If the generator is just barely large enough, start the well pump before the refrigerator and other motors.

Save water, save fuel

The pump runs only when the pressure tank drops. Running the generator in blocks and avoiding laundry during outages reduces pump cycles. If you have a large pressure tank, you may get several flushes and a shower between pump runs.

Try your numbers in the generator size calculator.

More on private wells

For how the pump, pressure tank, and well fit together, see how private wells work. If a storm also flooded the wellhead, follow after a flood: your septic system and well before you drink the water.

Why well pumps are hard to start

A submersible or jet pump is an induction motor, and induction motors draw a large inrush current for a fraction of a second when they start. That surge can be several times the running current. A generator that can easily supply the running watts may still stall, trip its breaker, or drop voltage so low that the pump hums but doesn't start. That's why pump sizing focuses on starting watts.

Two-wire vs. three-wire submersibles

Submersible pumps come in two main types:

• Two-wire pumps have the starting components built into the motor. There's no control box in the house, and starting current can be high.
• Three-wire pumps use a control box mounted in the house or well house, which contains the starting capacitor and relay. Look for the box near the pressure tank. Its label usually shows the horsepower and voltage.

Either way, look for the motor's horsepower, voltage, and current figures on the control box, the pump's documentation, or the original installer's records. Some newer systems use variable-speed (constant-pressure) drives, which ramp the motor up gradually and can greatly reduce starting surge. That makes them much easier on generators.

Typical sizes by horsepower

Exact figures depend on the motor, but common rule-of-thumb generator sizes for submersible well pumps are roughly:

Pump horsepower	Typical running watts	Typical starting watts
1/2 hp	about 1,000	about 2,000 to 3,000
3/4 hp	about 1,500	about 3,000 to 4,500
1 hp	about 2,000	about 4,000 to 6,000
1.5 hp	about 2,500 to 3,000	about 6,000 to 7,500

Treat these as starting points. If you can find the motor's locked-rotor amps on its documentation, you can estimate the surge more precisely: multiply the locked-rotor amps by the voltage.

Connecting a well pump to a generator

Most well pumps are hard-wired at 240 volts, so you can't just plug them into an extension cord. They need:

• a generator with a 240-volt output, usually a four-prong L14-30 or a 50-amp outlet;
• a transfer switch or interlock installed by a licensed electrician, so the pump circuit can be powered safely without backfeeding the utility.

See our guide on transfer switches vs. interlocks.

Planning around the pressure tank

Your pressure tank stores a small amount of pressurized water, often just a few gallons usable between pump cycles. During an outage, you can reduce how often the pump starts by:

• filling bathtubs, pitchers, and buckets while the generator is running;
• avoiding running the pump at the same time as other big motor loads, such as the furnace blower or AC compressor;
• letting the pump finish its cycle before starting another large load.

If the pump runs constantly or short-cycles, check the tank's air precharge. An incorrect precharge makes the pump start more often, which is harder on both the pump and the generator.

Water for drinking and sanitation

Keep a few days of stored drinking water as a backup in case the generator fails or runs out of fuel. Ready.gov recommends storing at least one gallon of water per person per day for several days, for drinking and sanitation. After flooding, private wells may become contaminated. If your wellhead was underwater, don't drink the water until the well has been disinfected and tested.