Generator size calculator
Add up the running watts of everything that will run at the same time, enter the running watts of the largest motor and how many times that it draws when starting, and choose a margin. The calculator gives the generator rating in kW and kVA that covers both steady running and the start.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Add up the running watts of every load that can run at the same time, including the motors.
- Enter the running watts of the largest motor (the one with the biggest surge) and its starting multiplier.
- Choose a safety margin for steady running and the generator’s power factor.
- Read the rating in kW and kVA, and the two figures it is based on.
How it works
- Running with margin = total running watts × (1 + margin).
- Starting peak = total running watts + largest motor × (multiplier − 1): everything else keeps running while the motor draws its starting power.
- Rating = the larger of the two.
- kVA = kW ÷ power factor.
Worked example
A 3,000 W total load that includes a 1,000 W pump with a 3× start, 20 % margin, power factor 0.8:
- running with margin: 3,000 × 1.2 = 3,600 W;
- peak while the pump starts: 3,000 + 1,000 × (3 − 1) = 5,000 W, of which the pump draws 3,000 W;
- rating: 5 kW, or 5 ÷ 0.8 = 6.25 kVA.
Tips and common mistakes
- Running and starting watts are different numbers; add running watts, then add one surge.
- Inverter generators often have little surge capacity beyond their rating; compare the peak with the surge rating on the datasheet.
- Altitude and heat reduce engine output; manufacturers give derating figures.
- Fixed installations connected to a building need a transfer switch and must follow local electrical rules.
Frequently asked questions
How do I size a generator?
List what will run at the same time and add the running watts. Then find the motor with the biggest starting surge: while it starts, the generator carries the other loads plus that motor's starting draw. Choose a generator whose rating covers the larger of the running load with a margin and that peak.
What starting multiplier should I use?
Take it from the appliance or motor data: some list starting watts, others locked-rotor current. Divide starting watts by running watts to get the multiplier. Resistive loads such as heaters and kettles have no surge, so their multiplier is 1.
Why is the generator rated in kVA?
The alternator is limited by current, which depends on apparent power. kW = kVA × power factor. Many generators are rated at a power factor of 0.8, so a 5 kW load needs 6.25 kVA.
Why start only one motor at a time?
Starting surges last a second or two. If motors start one after another, the generator only has to handle the biggest single surge on top of what is already running. Starting several together adds their surges.