Calculyn

Generator Size Calculator

Find what size generator you need by adding up the watts of the appliances you want to run.

Results update as you type.

Suggested generator size

5,000watts
Running watts
2,400W
Peak starting watts
4,000W
Safety margin included
20%
Suggested generator size5,000

Estimates only — check each appliance's actual label and consult an electrician for permanent installations.

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Quick answer

Fridge + furnace + lights + 500W of extras = 2,400 running watts; with the furnace surge, a 6,000W generator covers it comfortably.

How it works

Appliances with motors need 2–3× their running watts for a few seconds at startup. Add up running watts, add the largest motor's extra starting surge, then choose a generator about 20% above that peak.

Formula

Size = (Running watts + Largest starting surge) × 1.2, rounded up

Assumptions & limitations

  • Typical wattages shown in each label
  • Largest motor starts while everything else runs
  • 20% safety margin

Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators

Frequently asked questions

Can I run a whole house on a portable generator?

Usually just the essentials. Whole-home coverage typically needs a 10,000W+ standby generator.

What if I undersize it?

The generator will overload and shut down when a motor starts, or appliances may run poorly.