Generator Sizing Calculator

Running load, W

Starting surge, W

Насос или компрессор при запуске берёт втрое больше.

Power factor

Headroom, %

Result

Recommended set5 кВА
Required, kVA4.5
Required, kW3.6
Peak load, W5,000
Peak, kVA6.25
Fuel, l/h1.26

Starting surge decides more than the running load

Works out the generator rating a load needs, allowing for motor starting surge, power factor and headroom, plus fuel use.

Adding up appliance wattages is not enough. Anything with a motor — pump, fridge, compressor — draws three to five times its rating at the moment it starts. A generator sized on running load simply stalls on that surge.

The second trap is kilowatts against kilovolt-amps. Generators are rated in kVA at a 0.8 power factor, so a "5 kVA" machine delivers 4 kilowatts. Do the arithmetic in the same units as the nameplate.

What this calculator shows

Required output in kilowatts and in kilovolt-amps
Peak load including the starting surge
The nearest standard size and a rough fuel consumption

What to keep in mind

Headroom defaults to twenty percent: running continuously at the limit shortens engine life sharply.
Fuel use is estimated at 0.35 litres per kilowatt-hour, an average for petrol sets.

FAQs

What starting surge should I allow?

Fridges and pumps draw three to five times their rating at start, compressors up to seven, angle grinders and saws about three. Heaters, lamps and electronics have no surge at all.

Why not load a generator to a hundred percent?

It is designed for continuous operation at around eighty percent of rating. Running at the limit overheats the engine and windings and cuts service life dramatically.

Inverter or conventional?

An inverter set gives a clean sine wave, runs quieter and is more economical at part load — needed for electronics and boilers. A conventional set is cheaper for the same output and fine for tools and heaters.

How much fuel does it use?

Roughly 0.3 to 0.4 litres per kilowatt-hour. A 5 kW set at full load burns about two litres an hour — nearly fifty litres a day if it runs continuously.

Worked example

Standby for a country house

Input: 3,000 W running, 2,000 W surge, 0.8 power factor, 20 percent headroom

Output: 4.5 kVA continuous, 6.25 kVA peak — an 8 kVA set

Note: The peak figure drives the choice. A 5 kVA machine would handle the running load but stall the moment the borehole pump started.

Useful links

All tools in Electrical

Generator Sizing Calculator

Result

Related tools

kVA to kW Calculator

Power Factor Calculator

Solar Panel Calculator

Useful links