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Calculation Machine

Generator Size Calculator

Watts needed to run your home.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Minimum generator size
8,000 W
Running watts
4,200 W
Peak demand
6,400 W
Fuel needed
9 gallons
In kilowatts
7.68

Quick answer

Size a generator on total running watts plus the largest single starting surge, then add 20% headroom.

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Total running watts4200 W
  • Largest starting surge2200 W
  • Runtime needed12 hours
  • Fuel use0.75 gal/hour

Result

  • Minimum generator size8,000 W
  • Running watts4,200 W
  • Peak demand6,400 W
  • Fuel needed9 gallons
  • In kilowatts7.68

How to use the Generator Size Calculator

  1. 1Add the running watts of everything you need.
  2. 2Find the largest single starting surge (usually a fridge or pump).
  3. 3Add 20% headroom.

Frequently asked questions

What are typical loads?

Fridge 700 W running, sump pump 800 W, window AC 1200 W, furnace fan 800 W.

Do I need a transfer switch?

Yes for hardwired circuits — never backfeed through an outlet.

About this calculator