Home › Generator Size Calculator

Generator Size Calculator

Add up running watts and the largest starting (surge) watts of the appliances you want to power to choose a generator size.

Fridge running W

Sump pump / well pump running W

Furnace fan running W

Lights & electronics W

Window AC running W

Other running W

Largest extra starting watts

Safety margin (%)

How to use it

Enter the running watts of everything that may run at once.
Enter the extra starting watts of the single largest motor (often 2-3× its running watts).
Add a 10-25% margin.

How it works

Required = (sum of running watts + largest starting surge) × (1 + margin). Use a transfer switch or interlock installed by a licensed electrician - never backfeed a house through an outlet.

FAQ

What size generator to run a house?

Essentials (fridge, sump pump, furnace fan, lights) usually need about 5,000-7,500 W; central AC and electric ranges push it much higher.

Related calculators

X Facebook Reddit WhatsApp LinkedIn

Embed this free calculator on your site (copy & paste):

<iframe src="https://kwhcalc.pages.dev/generator-size-calculator/?embed=1" width="100%" height="640" style="border:1px solid #ddd;border-radius:12px" loading="lazy" title="KWhCalc calculator"></iframe><p style="font-size:13px">Calculator by <a href="https://kwhcalc.pages.dev/generator-size-calculator/">KWhCalc</a></p>

Free calculator by KWhCalc