What size generator do I need for a well pump?

No power means no water if you're on a well. A well pump is also the hardest common load for a portable generator. It's a big motor, it's often 240 V, and it's usually hard-wired. Here's how to size for it without guessing.

How many watts does a well pump use?

Once it's running, not as many as you might think. Franklin Electric, one of the biggest makers of submersible well-pump motors, publishes the motor data. For its standard 4-inch, 230 V motors:

Pump size	Running watts (full load)	Running watts (max load)
1/2 HP	670	960
3/4 HP	940	1,310
1 HP	1,210	1,600
1.5 HP	1,710	2,280

Our calculator uses 1,000 running watts for a 1/2 HP pump. Champion's chart agrees, listing 750–1,000 W.

Why the generator must be bigger than the pump

The start is the hard part. A well pump starts under a long column of water, and the motor pulls a huge current for a moment. Franklin's manual says a generator must hold at least 65% of the motor's rated voltage during the start. Otherwise the motor may not have the torque to get going.

That's why Franklin gives a minimum generator size for each motor, not just a watt figure:

Pump size	2-wire pump	3-wire pump
1/3 HP	2,250 W	1,500 W
1/2 HP	3,000 W	2,000 W
3/4 HP	4,500 W	3,000 W
1 HP	6,000 W	4,000 W
1.5 HP	7,500 W	5,000 W
2 HP	—	7,500 W

These are Franklin's kW ratings for the generator. Champion's chart is gentler, at 1,500–2,000 starting watts for 1/2 HP and 2,000–4,000 for 1 HP. We go with the motor maker. Our calculator uses 3,000 starting watts for a 1/2 HP pump, so it will start either type.

2-wire or 3-wire? A 3-wire pump has a separate control box on the wall in your house, usually near the pressure tank. A 2-wire pump has no control box. Its starting parts are down in the well.

A worked example: 1/2 HP well pump, fridge and lights

• Running: 1,000 (pump) + 400 (fridge) + 100 (lights) = 1,500 W
• Biggest surge: the pump, 3,000 − 1,000 = 2,000 W
• Starting: 1,500 + 2,000 = 3,500 W
• With 20% headroom: about 1,800 running and 4,200 starting watts

So plan on a generator of about 4,000 W or more with a 240 V outlet. A 5,000–7,500 W unit leaves room to add a furnace fan or a sump pump. The calculator above is pre-filled with this example.

Is my well pump 120 V or 240 V?

Most submersible well pumps are 240 V. In Franklin's standard 4-inch line, only the 1/2 HP motor comes in a 115 V version; everything bigger is 230 V. Some shallow-well jet pumps run on 120 V or can be wired for either. To check:

• Look at the breaker in your panel. A 240 V pump uses a double breaker (two handles tied together).
• Read the label on the control box, pressure switch cover or jet pump motor.
• If in doubt, ask an electrician. Don't open the pressure switch cover with the power on.

What a 240 V pump needs from the generator

A 240 V pump can't run from the generator's normal 120 V outlets. You need:

1. A generator with a 240 V outlet. The usual one is the 4-prong L14-30. It carries 30 A at 120/240 V, which is 7,200 watts.
2. A transfer switch or interlock, installed by a licensed electrician. It connects the generator to the pump's circuit safely.
3. A matching cord and inlet box. See 30 amp vs 50 amp generator cords.

Never backfeed the house through a dryer outlet or a wall outlet with a "double male" cord. It can electrocute a utility worker and start a fire. Our interlock vs transfer switch guide explains the safe options.

Tips from the motor maker's manual

Franklin's manual gives some useful rules for running a well pump on a generator:

• Start the generator first, then turn on the pump. Turn the pump off before you shut the generator down.
• Don't let the generator run out of fuel with the pump running. Franklin warns the pump's bearing can be damaged as the generator coasts down.
• Derate at high altitude. Generators make less power up high, so size up if you live in the mountains.

Will a 3,500 watt generator run a well pump?

It can run a 1/2 HP pump if it has a 240 V outlet, or if your pump is 120 V. It's cutting it fine for a 2-wire pump, which needs 3,000 W, and has little room for anything else. See what a 3,500 watt generator can run.

Check your own pump

Find the HP and volts on the control box, the pressure tank's paperwork or the well driller's records. The driller's invoice often lists the pump model. Then use the two tables above.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.