What size generator do I need for a well pump?
A 1/2 HP well pump needs a generator rated about 3,000 running watts. A 1 HP well pump needs about 6,000 running watts.
If you also want to run the usual outage loads at the same time, a refrigerator, freezer, lights, wifi, and phone chargers add 720 running watts and 1,620 peak. That raises the requirement to 3,900 watts for a 1/2 HP pump and 6,900 watts for a 1 HP pump.
How many watts does a well pump use?
Running watts are what the appliance draws continuously. Starting watts (also called surge watts) are the brief spike a motor needs for a second or two when it starts. Needs alone is what that appliance requires by itself. With basics adds a refrigerator, a chest freezer, 20 LED bulbs, wifi, and phone chargers, which together draw 720 running watts and 1,620 peak.
Both need figures are the peak load with a 20% safety margin, rounded up to the nearest 100W. The recommended class is the smallest generator whose continuous rating covers the with-basics figure.
Well Pump (½ HP)
- Running watts
- 900W
- Starting watts
- 2,500W
- Needs alone
- 3,000W
- With outage basics
- 3,900W
- Recommended class
- 5,300W or more
Well Pump (1 HP)
- Running watts
- 2,000W
- Starting watts
- 5,000W
- Needs alone
- 6,000W
- With outage basics
- 6,900W
- Recommended class
- 10,500W or more
Why does a well pump need a 240 volt generator?
Most submersible well pumps run on 240 volts, not 120. They use both legs of your electrical panel so the current stays low enough for the wire that runs down the well.
That matters when you buy a generator, because a generator can have plenty of watts and still not run your pump if it has no 240 volt outlet. Check for an L14-30R receptacle or a 120/240V switch. A generator with only 120 volt outlets (an L5-30R or 5-20R) cannot run a 240 volt pump no matter its rating.
Of the two sizes above, the Westinghouse 6500 Watt Dual Fuel (WGen6500DF) and the DuroMax XP13000HX Dual Fuel both have a 240 volt outlet. Worth confirming on the unit you buy, since not every portable does.
How is this calculated?
Two rules decide everything on this page:
- Total running watts must stay under the generator's continuous rating.
- Total running watts plus the single largest surge must stay under the generator's peak rating.
For the second rule you add only the largest surge, not every surge, because appliances rarely start at the same instant. We then add a 20% margin so the generator is not running flat out.
Appliance wattages are typical values for common residential models. Your specific appliance may differ. The nameplate on the appliance itself is always the better number.
What else should you know before you buy?
- The startup surge is the real limit. A 1/2 HP pump draws only 900 watts while running, but needs 2,500 watts for the moment it starts. A 1 HP pump goes from 2,000 running to 5,000 starting. If you size on running watts alone you will buy a generator that trips every time the pump kicks on.
- No power means no water on a private well. Your pressure tank holds a small reserve, worth roughly a few flushes and one short shower, and then you are out. Unlike city water, a well gives you nothing until the pump has electricity.
- Deep wells and high head need more. The horsepower on the pump label is the starting point. A pump lifting water a long distance works harder, so it draws more than the typical figure here.
- Have an electrician make the connection. Wiring a pump circuit to a generator, or installing a transfer switch, is electrical work with real risk. Use a licensed electrician rather than an extension cord.
Add up your own appliances
Pick what you want to power and get your exact size, with the math shown.
Open the Generator Size Calculator →Frequently asked questions
What size generator do I need for a 1/2 HP well pump?
About 3,000 running watts for the pump by itself. If you want to run a refrigerator, freezer, lights, wifi, and chargers at the same time, plan on 3,900 watts.
What size generator do I need for a 1 HP well pump?
About 6,000 running watts for the pump alone, or 6,900 watts with the usual outage loads added. That puts you in the larger portable range.
Will a generator run my well pump if it only has 120 volt outlets?
No. Most submersible well pumps are 240 volt appliances. The generator needs a 240 volt outlet, usually an L14-30R receptacle, or a 120/240V selector. A generator with only 120 volt outlets cannot run the pump no matter how many watts it produces.
Why does my well pump need so much more than it uses?
Because of the starting surge. A 1/2 HP pump draws about 900 watts running but needs roughly 2,500 watts to start. A 1 HP pump goes from about 2,000 running to 5,000 starting. The surge lasts a second or two, but the generator has to supply it.
Do I get any water when the power goes out?
A little, and only briefly. Your pressure tank holds a small reserve, which is usually enough for a few flushes and one short shower. After that a private well is dry until the pump has power again.
Related
- Generator size calculator (add up your own appliances)
- Appliance wattage chart (running and starting watts)
- What will a 7,500 watt generator run? (a full worked example)
- Portable vs standby generator (which type fits your situation)
Last reviewed September 2026. Appliance wattages are typical residential values and may differ from your specific models.