Generator Size Calculator: Running and Starting Watts, kVA

Size a generator in watts and kVA from your appliance list, with motor starting surges, power factor, a reserve margin and the best start order.

How to size a generator

Add up the rated watts of everything that will run together and add a margin: that is the continuous output. The generator must also absorb the surge of the largest motor, started last. Divide kW by the power factor (0.8 on most datasheets) to get kVA. A fridge, a freezer, a 750 W sump pump and some lights need about 2 kVA continuous and 3.5 kVA at start-up.

The method

  • Running load = sum of rated watts × quantity.
  • Continuous output = running load × (1 + margin).
  • Staggered starting peak = running load + (starting multiple − 1) × watts of the largest motor.
  • kVA = kW ÷ cos φ.

Motors draw 2 to 3 times their running current for a few seconds, and Maytag advises doubling a refrigerator's wattage (Maytag guide). The Robin catalogue (2012) gives 2 to 4 times for drills, saws and compressors.

Worked example

Fridge 150 W (× 2), freezer 150 W (× 2), sump pump 750 W (× 3), boiler 120 W, six 10 W LEDs and a 40 W router: 1,270 W running. With a 25% margin: 1,588 W, or 1.98 kVA at cos φ 0.8. The pump, started last, adds 2 × 750 = 1,500 W: a 2,770 W peak, or 3.46 kVA. Starting everything at once would reach 3,070 W.

Outages in Germany: BBK advice

The BBK brochure (January 2019) rules out indoor use because of exhaust gases and requires an electrician and a changeover switch to feed the house wiring.

Limits

Starting multiples are rules of thumb; the nameplate wins. Three-phase loads and fuel use are not covered. For battery backup, see the UPS runtime calculator.

Frequently Asked Questions

What size generator do I need for a house?

For outage essentials (fridge, freezer, boiler, lights, router), the default example gives 1.27 kW running, 1.98 kVA continuous and 3.46 kVA with a pump surge. Each heating appliance adds 1,500 to 2,000 W according to the BBK. For a whole house, the BBK advises having an electrician size the system from the appliances used at the same time.

How do I convert kW to kVA?

kVA = kW ÷ cos φ. Generators are usually rated at cos φ 0.8, so 4 kW equals 5 kVA. An inverter generator rated at cos φ 1 shows the same figure in kW and kVA.

Why start the largest motor last?

The generator then only has to supply one surge on top of the loads already running. In the default example the peak drops to 2,770 W instead of 3,070 W when everything starts together.

Can I plug a generator into a wall outlet?

No. The German BBK requires an electrician and a manual or automatic changeover switch to feed the house wiring, and large loads such as the cooker must be switched off first. Portable appliances connect to the generator with an extension cord.

Is this tool really free?

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