FigureDesk

Generator Size Calculator

Calculate what size generator you need from your appliance list — total running load, the worst-case starting surge, and a suggested generator size.

Your numbers

Appliance 1
W

From the appliance nameplate

W

Leave blank if same as running watts

%

A planning cushion you choose — not a legally required safety factor

Result

Add at least one appliance with a running wattage to calculate.

How it works

Add every appliance you want the generator to run, with its running and starting (surge) watts. Motors, compressors, and pumps draw much more power for a fraction of a second at startup than they do once running. The generator has to be able to supply the worst moment — the single largest starting surge that can happen while everything else you're already running keeps running — not the sum of every appliance's surge added together, which would produce an unrealistically oversized recommendation.

running total = Σ(running watts × qty) for appliances running continuously
worst-case start = max over appliances of (starting watts + everything else still running)
planning margin load = running total × (1 + margin / 100)
required capacity = max(worst-case start, planning margin load)

Worked example

A refrigerator (150 W running, 600 W starting) and LED lighting (100 W) running continuously, plus an occasional-use sump pump (800 W running, 2,000 W starting):

  • Running total: fridge + lights = 250 W
  • Worst-case starting requirement: the sump pump starting while the fridge and lights keep running = 2250 W
  • Required capacity (with a 20% planning margin): 2250 W
  • Suggested generator size: 3,500 W

Assumptions and advisories

  • Appliance wattage should come from the appliance's own nameplate or manual — this calculator has no built-in appliance library and does not guess typical values.
  • Generator starting capability varies by manufacturer and model — the suggested size is not a guarantee that any specific generator will start every appliance you list.
  • High altitude can reduce generator output — this calculator does not calculate an altitude derating. Consult your generator manufacturer's derating chart.
  • Temperature can affect generator performance — this calculator does not calculate a temperature derating. Consult the manufacturer's specifications.
  • Fuel type, fuel consumption, and runtime on a tank are not calculated here — check the manufacturer's specification for your chosen fuel type.
  • Standard sizes shown are common commercially available ratings, not a regulatory standard or a manufacturer recommendation.
  • Transfer-switch installation should follow applicable local requirements and be performed by a qualified professional where required.
  • This does not check code compliance, electrical permitting requirements, or wire/breaker sizing for the installation.

FAQ

Why is the starting requirement not just the sum of every appliance's starting watts?

In practice, appliances essentially never all start in the same instant — sizing for the sum of every possible surge produces a wildly oversized (and expensive) generator for a scenario that doesn't happen. Sizing for the single worst simultaneous start — the largest surge while everything else you're running keeps running — is the standard, defensible approach.

Does this account for altitude or temperature derating?

No. Generator output derating by altitude and temperature is manufacturer- and model-specific, and this calculator doesn't have a sourced, citable default to apply — rather than guess, it leaves that entirely to your generator's own manufacturer specification, which you should check if you're at meaningfully high altitude or extreme temperature.

What's the difference between this and the Inverter Size Calculator?

They use the same running/starting-load math, but size different equipment: this sizes a fuel-powered generator; the Inverter Size Calculator sizes a battery-powered DC-to-AC inverter, which has different surge characteristics.

Is the suggested generator size guaranteed to work?

No. This is a calculated estimate based on the load you enter. Actual starting performance depends on the specific generator's design, its own surge rating, altitude, temperature, and fuel type — verify against the manufacturer's own specifications before buying.

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FigureDesk results are advisory and do not replace a licensed electrician, a qualified engineer, or the applicable local electrical code. Verify any electrical work against your local code and with a qualified professional before relying on it.