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Energy & Appliances

Generator Size Calculator

Estimate a generator wattage target from total running load and the largest startup surge.

Free•Result emailed + shown•Reviewed September 9, 2026
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Step 1

Enter your numbers

Method

How this generator size calculator works

Running watts and the largest additional startup surge are added, then headroom is applied to create a conservative generator rating target.

How to use it

  1. Combined running watts. Enter the value in W.
  2. Largest additional startup surge. Enter the value in W.
  3. Headroom. Enter the value in %.
  4. Request the result. Add your email, then select “Email & show my result.”

What to do with the number

This is a load-sizing estimate, not an electrical installation design. Transfer equipment, voltage, phase and code requirements still matter. Use actual appliance running and starting watts where possible; motor-driven equipment can have large startup surges.

Planning estimate

Use this result as a starting point, not a guarantee. Product specifications, site conditions, manufacturer guidance, local codes, taxes, labor, tolerances, and rounding can change the real requirement or cost.

FAQ

Questions about this calculator

What does the Generator Size Calculator calculate?

Estimate a generator wattage target from total running load and the largest startup surge.

What inputs does this generator size calculator use?

The calculator uses combined running watts, largest additional startup surge, headroom. You can replace every default with the numbers that match your situation.

How should I use the Generator Size Calculator result?

This is a load-sizing estimate, not an electrical installation design. Transfer equipment, voltage, phase and code requirements still matter.

What can make the real-world result different?

Use actual appliance running and starting watts where possible; motor-driven equipment can have large startup surges.

Can I change the calculator assumptions?

Yes. The editable inputs are the assumptions. Replace the default values with your own measurements, prices, rates, quantities, or specifications before requesting the result.

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