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Generator Size Calculator

Enter the running watts of the lights and electronics, of other appliances, and of your largest motor such as a well pump or air conditioner. The calculator adds a safety margin and the starting surge to give the generator size you need.

Your numbers

2 to 3 is common for motors. Use the starting watts on the nameplate when you can.

Result

How we calculate it

Generators have two ratings: running (continuous) watts and starting (surge) watts. Add the running watts of everything you want to power at the same time. That is your running load.

Motors draw several times their running power for a moment when they start. The generator must cover the running load plus the extra the largest motor needs: running watts × (starting multiplier − 1). This is the starting load.

A safety margin keeps the generator from running at its limit. The running rating needed is the running load times (1 + margin). Find a generator whose running watts are above that and whose starting watts are above the starting load.

Assumptions and limits

  • This is a planning estimate, not electrical advice. Wiring must follow the electrical code adopted where you live, and a licensed electrician or permit inspector decides what is safe for your job.
  • Start with the wattage printed on each appliance label. Where only amps are given, multiply by the voltage. Appliances that will not run at the same time need not be added together.
  • Fuel type, altitude, heat and the transfer switch or inlet you will use change what you need. Have an electrician install the connection to your home.

Frequently asked questions

What size generator do I need to run a house?

It depends on what you power. Essentials such as a refrigerator, lights, a furnace fan and a sump pump often need 3,000 to 7,500 watts; central air conditioning or an electric range can push it much higher. Add up your own list.

What are starting watts?

The brief surge a motor draws when it starts, often two to three times its running watts. The generator must be able to supply it on top of the other loads.

Why add a safety margin?

Running a generator at its limit shortens its life and leaves no room for loads you forgot. A margin of about 20% is common.

Can I run everything at once?

You can, if the generator is big enough. Many people instead list essentials only and avoid starting big motors at the same time, which allows a smaller generator.

How do I convert watts to amps?

Divide the watts by the voltage. 3,600 W at 120 V is 30 A, and the same load at 240 V is 15 A.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: Lights and electronics (running watts) = 1000; Other appliances, not counting the largest motor (running watts) = 2500; Largest motor (running watts) = 1000; Motor starting multiplier = 3; Safety margin (%) = 20.

  • Generator running watts needed 5,400 W
  • Running load 4,500 W
  • Peak starting load 6,500 W
  • Starting watts the generator must supply 6,500 W
  • Running amps at 120 V 37.5 A
  • Running amps at 240 V 18.8 A

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.