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Generator sizing calculator

Tick everything that may run at the same time. You get two targets: the continuous watts to carry the load, and the surge watts to start the largest motor.

UseApplianceRunning WExtra start W

Example values only. "Extra start W" is the surge above running watts, not the full starting figure.

A margin for loads you forgot and labels that understate.
Total running load
—
Continuous rating target
—
Starting (surge) target
—

Compare both targets with the manufacturer's rated (running) and peak (starting) watts.

Generator safetyRun a fuel generator outdoors only, never in a garage, shed or basement. US and Canadian safety agencies say to keep it at least 20 feet (6 m) from the house with the exhaust pointing away. Never plug a generator into a wall outlet.

How to use it

  1. Tick what will run together. "Essential loads" selects a fridge, a freezer, the modem and router, and LED lights.
  2. Replace the example watts with figures from your appliance labels or manuals.
  3. Enter extra starting watts for motors. If a manual gives 1,200 W starting and 700 W running, the extra is 500 W.
  4. Keep headroom at 20% unless you have measured every load.

The formula

continuous target = total running watts × (1 + headroom)

surge target = (total running watts + largest extra start watts) × (1 + headroom)

With the essential loads, running watts are 180 + 150 + 20 + 40 = 390 W. The continuous target is 390 × 1.2 = 468 W, shown as 500 W. The largest extra surge is the fridge at 500 W, so the surge target is 890 × 1.2 = 1,068 W, shown as 1,100 W. Results round up to the next 100 W.

Reading the result

TargetCompare it withIf the generator falls short
Continuous rating"Rated" or "running" watts on the generatorIt overloads and trips, or the voltage sags and appliances misbehave.
Starting (surge)"Peak", "starting" or "surge" wattsThe motor stalls or fails to start even though the running load fits.

The number in a generator's model name is usually its peak watts. Shop by the rated figure.

Why only the largest surge is added

Motors draw their surge for a second or two. It is unlikely that the fridge, the freezer and the sump pump all start in the same instant, so the calculator adds only the biggest one. If you want to be sure, start the large motors one at a time, or add the second-largest surge to your target by hand.

Limits

  • Watts tell you nothing about how long the fuel lasts. Use the generator fuel planner for that.
  • Well pumps, central air conditioners and electric water heaters run on 240 V and need a generator with a 240 V outlet and a transfer switch installed by a licensed electrician.
  • Sensitive electronics are happier on an inverter generator, which produces cleaner power than a conventional open-frame unit.

Common questions

Where do I find starting watts?

In the appliance manual or on the manufacturer's support page, sometimes listed as locked-rotor amps (LRA). Multiply LRA by the voltage for a rough upper limit.

Does this work for a power station too?

Yes. A power station has the same two ratings: continuous AC output and surge. Then check its watt-hours with the battery runtime calculator.

Is a bigger generator always better?

No. An oversized generator burns more fuel at light load, costs more and is louder. Aim to run at roughly half to three-quarters of its rated output.

Can I plug the generator into a wall outlet?

Never. That sends power back into the utility lines and can kill a line worker or a neighbour. Use extension cords to individual appliances, or have a transfer switch or interlock fitted.

Related

Generator fuel planner →Generator safety →Power station or generator? →

Reviewed October 2026. Results are planning estimates. Check them against your equipment's specifications. Report a problem.