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Generator Size Calculator

Work out the generator size you need — from your loads and their starting multipliers — in running watts, starting watts, kW and kVA.

A generator is sized by its starting watts, not its running watts — and motors are the reason. This calculator adds the loads, finds the biggest start, and sizes the generator for it.

Enter the loads; the generator size follows in kW and kVA.

The first load's running watts (e.g. a fridge, 150 W).
Motors need 2–3× their running watts to start. Resistive loads (lights, heaters) use 1.
The second load's running watts (e.g. an air conditioner, 1,200 W).
Motors need 2–3× their running watts to start.
The third load's running watts (e.g. lights, 200 W).
Resistive loads use 1.
Extra headroom on top of the starting watts — 20% is a common planning figure.

Formula

running watts = sum of load watts starting watts = running watts + (largest starting watts − largest running watts) — only the biggest motor starts at once; the rest are already running. recommended kW = starting watts × (1 + margin%) ÷ 1,000 recommended kVA = kW ÷ 0.8 (power factor 0.8)

All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.

How to use this calculator

  1. Enter each load's running watts and starting multiplier.
  2. Set the safety margin.
  3. Read the running watts, the starting watts and the recommended size in kW and kVA.

Worked example

LoadsFridge 150 W ×3, AC 1,200 W ×2.5, lights 200 W ×1
Running watts150 + 1,200 + 200 = 1,550 W
Starting watts1,550 + (3,000 − 1,200) = 3,350 W
Recommended (20% margin)3,350 × 1.20 = 4,020 W = 4.02 kW
kVA4.02 ÷ 0.8 = 5.02 kVA

Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.

Frequently asked questions

Why does a generator need starting watts?

Because motors draw 2–3 times their running watts for a few seconds when they start. A generator sized only for running watts will stall when the fridge or AC kicks in.

Why only the biggest motor?

Because motors rarely start at the same instant. The method sizes for the biggest motor starting while everything else is already running — the standard planning approach.

What is the difference between kW and kVA?

kW is real power; kVA is apparent power. Generators are rated in kVA; dividing kW by a power factor of 0.8 gives the kVA. A 4.02 kW load needs about a 5.02 kVA generator.

Is this a substitute for electrical design?

No. This is a sizing estimate for planning. Generator and load selection require qualified professionals and local code compliance — see the safety notice.

Related calculators

Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.