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Energy Calculators

Generator Size Calculator

Verified formula Updated Jun 2026 Private — runs on your device

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W
W
Verified formula Private

Generator size needed

4,500

Approx. kVA (PF 0.8)
5.63

For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.

How to use the Generator Size Calculator

The Generator Size Calculator works out your generator size needed, along with 1 related figure in an instant. Enter total running watts and additional starting (surge) watts and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.

  1. Enter the total running watts.
  2. Enter the additional starting (surge) watts.
  3. Read off your generator size needed, together with approx. kva (pf 0.8) — the calculator updates automatically, with no button to press.

Formula

The Generator Size Calculator uses the formula:

Generator size needed = Total running watts + Additional starting (surge) watts

Worked example

For example, with total running watts of 3,000 W and additional starting (surge) watts of 1,500 W, the generator size needed is 4,500.

Inputs used
Total running watts 3,000 W
Additional starting (surge) watts 1,500 W
Results
Generator size needed 4,500
Approx. kVA (PF 0.8) 5.63

Results are estimates for educational use, not professional advice.

Frequently asked questions

Add the running watts of all appliances and the largest starting surge. 3000 running plus 1500 surge needs at least a 4500 W generator.

Motors in fridges, pumps and air conditioners draw extra power for a moment when starting. The generator must handle this surge.

Generators are often rated in kVA. Dividing watts by a power factor of about 0.8 and by 1000 gives the kVA needed.

Yes. Sizing a little above your peak need avoids overload and leaves room to add appliances later.

The Generator Size Calculator uses the formula: Generator size needed = Total running watts + Additional starting (surge) watts. For example, with total running watts of 3,000 W and additional starting (surge) watts of 1,500 W, the generator size needed is 4,500.

Enter the total running watts. Enter the additional starting (surge) watts. Read off your generator size needed, together with approx. kva (pf 0.8) — the calculator updates automatically, with no button to press.

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