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Generator Size Calculator

Sizes a generator from your running load and the largest starting surge, with altitude derating and an estimate of run time per tank.

W

Everything that will run at the same time. Add nameplate ratings, not guesses.

W

The biggest motor start, not the sum of them — motors rarely start simultaneously.

%

Generators lose output with altitude and heat, and should not run at 100% continuously.

m

Naturally aspirated engines lose roughly 3% of output per 300 m above sea level.

L/hr

Optional, from the spec sheet. Used to estimate run time and consumption.

L
Continuous rating needed
— W

Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

Peak rating needed — W
Nearest common generator —
Rating after altitude derating — W
Run time on a full tank — hrs
Sizing notes —

The formula

continuous = running watts × (1 + headroom) ÷ altitude factor peak ≥ largest single motor surge

Generators are quoted with two figures — running and peak — and both have to be satisfied. The common mistake is adding every motor's starting surge together; in practice they never all start at the same instant, so only the largest matters.

What to watch out for

Four things worth getting right:

  • Add running watts, take the largest surge. Motors do not all start together, so summing every surge oversizes the generator badly.
  • Do not run at 100%. Continuous operation near the rating shortens engine life and leaves nothing for an unexpected load. 20% headroom is standard.
  • Altitude costs output. Naturally aspirated engines lose about 3% per 300 m. At 1,500 m a 5 kW generator behaves like a 4.25 kW one.
  • Bigger is not automatically better. A lightly loaded diesel wet-stacks — unburnt fuel glazes the bores. Aim to run at 50–75% of rating.

Frequently Asked Questions

What size generator do I need?

Add the running watts of everything that runs at once, add 20% headroom, then check the peak rating covers your largest motor start. 3,500 W of load typically means a 5 kW generator.

What is the difference between running and starting watts?

Running watts are the steady draw; starting watts are the brief surge when a motor spins up, often 3–7× higher. Generators quote both, and both must be satisfied.

Do I add up all the starting watts?

No — only the largest. Motors do not start at the same instant, so add all the running watts and then add only the biggest single surge on top.

How does altitude affect a generator?

Thinner air means less oxygen and less power. Naturally aspirated engines lose roughly 3% of output per 300 m of elevation, so high-altitude sites need a larger unit.

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