Generator Size Calculator
The generator rating needed for a given running load plus the surge of the largest motor, in W and kVA.
Results
What this tool does
Sizing a generator is not a matter of adding up nameplate wattages. Resistive loads — lights, kettles, heaters — take exactly what they say. Motors take several times as much for the instant they start, and it is that instant that decides whether the machine copes. Add the running total, add the surge of the single largest motor, allow a margin for what gets plugged in later, and convert to kVA at the power factor of the load.
Formula
power = (running power + starting peak) × margin ; kVA = W ÷ power factor
Variables
| Symbol | Meaning | Unit |
|---|---|---|
rw | Running load | W |
sw | Starting surge of the largest motor | W |
mg | Safety margin | % |
pf | Power factor | — |
SZ | Generator rating | W |
KV | Generator rating | kVA |
PK | Peak demand | W |
KW | Generator rating | kW |
CU | Current | A |
Worked example
- Running load3500 W
- Starting surge of the largest motor2200 W
- Safety margin25 %
- Power factor0.8
- Generator rating7125 W
- Generator rating8.906 kVA
- Peak demand5700 W
- Generator rating7.125 kW
- Current38.72 A
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
Frequently asked questions
Why do starting watts matter so much?
Because anything with an induction motor — a fridge, a pump, a compressor, a circular saw — draws several times its running current for the first fraction of a second, and a generator that cannot supply that surge will simply stall or trip. Only the largest single motor's surge needs adding, since two motors rarely start in the same instant, and staggering the ones you can is the cheapest way to shrink the generator you need.