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Construction & Homeenergy

Generator Sizing Calculator

Generator sizing with motor starting surge. Sizing on running load alone is the classic error: a motor draws several times its rated power for a second or two on start, and a generator that cannot supply it will stall.

Also called: genset sizing calculator, generator kva calculator.

W
W
%
litres/kWh
Generator rating
10.5

10.5 kVA, sized on a peak of 7,000 W rather than the 4,000 W running load, because the largest motor draws 3 times its rating on start. With 20% headroom that is 8.4 kW. Running cost is about 114 an hour at full load. Only the largest motor is assumed to start at once. Two large motors starting together would need more.

In kilowatts
8.4
Peak demand
7,000
Motor starting surge
3,000
Fuel use per hour
1.2
Cost per hour
114
On motor starting
Only the largest motor is assumed to start at once. Two large motors starting together would need more.
On regulation
This is a sizing estimate. The applicable wiring or building regulation governs and overrides any figure here.
How this is calculated
Method and background

How this is calculated

The peak demand is the running load plus the extra current the largest motor draws while starting, typically three times its rating for an induction motor. Only the largest motor is assumed to start at once, which is the standard convention. Generators are rated in kVA and loads in kW, related by the power factor, so a 0.8 power factor means a 4 kW load needs 5 kVA. Headroom above that covers load growth and avoids running at full rating continuously.

only the largest motor is assumed to start at once, which is the standard convention
k
Starting multiplier

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a 4 kW load with a 1.5 kW motor

Total running load
4,000 W
Largest motor load
1,500 W
Motor starting multiplier
3
Power factor
0.8
Headroom above peak
20%
Fuel use
0.3 litres/kWh
Fuel price
95

Generator rating10.5

1500 x 2 surge; 7000 x 1.2 / 1000 / 0.8

Open this example

no motor means no surge

Total running load
4,000 W
Largest motor load
0 W
Motor starting multiplier
3
Power factor
0.8
Headroom above peak
20%
Fuel use
0.3 litres/kWh
Fuel price
95

Generator rating6

boundary

Open this example

Method and limits

What it assumes

  • One motor starting at a time, and a direct-on-line start.

What it deliberately does not model

  • Soft starters and variable frequency drives reduce the surge substantially.
  • Altitude and temperature derate a generator, often by ten percent or more.
  • Non-linear loads like inverter air conditioners behave differently again.

Formula version 1.0.0 · definition 1.0.0 · India · Report a problem with this calculator

Frequently asked questions

Why is the generator bigger than my total load?
Because motors draw several times their rated power for a moment while starting. A generator sized on running load stalls when the compressor kicks in.
What is the difference between kVA and kW?
The power factor. kW is real power and kVA is apparent power, so at a 0.8 power factor a 4 kW load needs a 5 kVA generator.