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Home & Garden
Generator Size Calculator
Size a generator from your running watts and largest starting surge.
Generator size
—
Recommended.
| Kilowatts | — |
|---|---|
| Running load | — |
| With headroom | — |
| — |
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Enter your figures above to see the step-by-step working.
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How the Generator Size Calculator Works
A generator has to cover two things: the steady running watts of everything switched on, and the extra surge that motors (fridges, pumps, air conditioners) draw for a second when they start. Add up the running watts of what you want to power, note the biggest single starting surge, and the generator needs to handle the running total plus that surge — with a little headroom so it isn’t maxed out.
This calculator recommends a size from your running load and largest surge, keeping at least 25% spare capacity. Check appliance labels for watts, or estimate volts × amps.
size = running watts + largest starting surge (min +25% headroom)
Frequently Asked Questions
What size generator do I need?
Add the running watts of everything you’ll power, then add the largest motor’s starting surge. This calculator sizes for that plus headroom.
What are starting watts?
The brief spike motors draw at start-up — often two to three times their running watts. The generator must cover the biggest one on top of the running load.
Why leave headroom?
Running a generator at 100% continuously shortens its life and leaves nothing for extra loads. Aim to use about 75–80% of its rated output.
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