Generator Sizing Calculator

Written byOleh Soloninupdated 25 September 2026

Running load, W

Starting surge, W

Extra power for starting the largest motor — a pump, compressor or fridge. The starting current is on the appliance's rating plate or datasheet.

Power factor

Headroom, %

Result

Generatorat least 6.3 kVA — the start-up decides
Required, kVA4.5
Required, kW3.6
Peak load, W5,000
Peak, kVA6.25
Working
3,000 × 1.2 ÷ 0.8 = 4,500 VA; (3,000 + 2,000) ÷ 0.8 = 6,250 VA

Starting surge decides more than the running load

Works out the smallest generator rating from the running load with headroom and from the start-up of the largest motor — and says which of the two decides.

A generator is sized on the larger of two figures. One is the running load with headroom, in kilovolt-amperes: 3,000 W × 1.2 ÷ 0.8 = 4,500 VA. The other is the peak at start-up, when the largest motor's starting surge adds to the running load: (3,000 + 2,000) ÷ 0.8 = 6,250 VA.

A generator smaller than the peak gets overloaded when the motor starts. So the calculator names the minimum on the larger figure — 6.3 kVA here — and says what set it: the start-up or the running load.

How to use

Add up the appliances that will run at the same time and enter the total as the running load.
Enter the starting surge of the largest motor — a pump, compressor or fridge — from its datasheet.
Set the power factor and headroom, then read the kW and kVA needed, the peak and the minimum generator rating.

Minimum rating for different loads

Each row is a separate run at a power factor of 0.8 and 20% headroom.

All rows come from the calculator on this page.
Running	Starting surge	With headroom	Peak	Generator at least
1,000 W	0	1.5 kVA	1.25 kVA	1.5 kVA
1,000 W	1,500 W	1.5 kVA	3.13 kVA	3.2 kVA
2,000 W	0	3 kVA	2.5 kVA	3 kVA
3,000 W	0	4.5 kVA	3.75 kVA	4.5 kVA
3,000 W	2,000 W	4.5 kVA	6.25 kVA	6.3 kVA
5,000 W	3,000 W	7.5 kVA	10 kVA	10 kVA

kW and kVA on the rating plate

Current heats a generator's windings, and apparent power in kVA sets the current. At a power factor of 0.8 a 5 kVA load draws 4 kW of real power; at 1, all 5 kW. That is why the calculator compares the load in kilovolt-amperes. The generator's plate also carries a kW limit, and the load must stay within it too: a 5 kVA / 4 kW set will not deliver 5 kW even at a power factor of 1.

What this calculator shows

Required output in kilowatts and in kilovolt-amps
Peak load including the starting surge
The minimum apparent power the generator needs, rounded up to 0.1 kVA

What to keep in mind

One power factor for the whole load; take it from the appliances' datasheets.
One motor starts at a time; if several can start together, add their surges.

FAQs

What size generator does a house need?

Add up what runs at once and the starting surge of the largest motor. For a 3,000 W running load and a 2,000 W pump start at a power factor of 0.8, at least 6.3 kVA.

What will a 5 kVA generator run?

At a power factor of 0.8 and 20% headroom, a running load of up to 3,333 W — and the running load plus the starting surge must stay within 4,000 W.

Why add headroom?

So the generator is not run at its limit and can take one more appliance switching on. The 20% in the calculator is an example; change it as you see fit.

What starting surge should I enter?

The difference between the starting and running power of the largest motor. It is on the appliance's datasheet. If only a starting current is given, starting current × voltage is the apparent power in volt-amperes; multiply it by the power factor at start to get watts, and subtract the running power.

How do a generator's kVA differ from kW?

kVA is apparent power and kW real power: kW = kVA × power factor. At 0.8 a 10 kVA generator gives 8 kW.

How much fuel does a generator use?

The calculator does not estimate it: consumption depends on the engine and its load, and the maker states it on the generator's datasheet.

Worked example

A house with a pump

Input: 3,000 W, 2,000 W starting surge, power factor 0.8, 20% headroom

Output: 4.5 kVA running, 6.25 kVA peak — a generator of at least 6.3 kVA

Note: The start-up decides: without the pump 4.5 kVA would do.

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Result

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