Generator Size Calculator
Find the generator size in kVA for your home or shop, allowing for the extra power motors draw when they start and some spare capacity.
Add up what you'll run at the same time: fans, lights, fridge, water pump, an AC.
Motors (AC compressor, pump, fridge) briefly draw 2 to 3 times their running power. Enter the extra for the biggest one.
0.8 is the standard rating for generators.
Running a generator at about 80% of its rating is easier on it and on fuel.
Generator size
7.5 kVA
- Exact need
- 7.5 kVA
- Peak load
- 5 kW
A 3,500 W load with a 1,500 W motor start needs about 7.5 kVA, so a 7.5 kVA generator.
How it's worked out
- Peak load
3,500 W running + 1,500 W starting5,000 W - In kVA
5,000 W ÷ 0.8 ÷ 1,0006.25 kVA - With spare capacity
× 1.27.5 kVA
- Inverter ACs start gently and need little extra; older ACs and water pumps need much more. Have an electrician check before buying, and never run a generator indoors.
The formula
kVA = (running watts + starting surge) ÷ power factor ÷ 1,000 × (1 + spare)
Worked example
A 3,500 W load plus a 1,500 W motor start is 5,000 W. Divided by 0.8 that's 6.25 kVA, and with 20% spare, 7.5 kVA.
Questions people ask
How do I calculate generator size in kVA?
Add your running load and the largest motor's starting surge in watts, divide by the power factor (0.8), divide by 1,000, and add about 20% spare.
What size generator do I need for a house?
Fans, lights, a fridge and a TV need about 2.5 to 3.5 kVA. Adding a water pump or an AC usually needs 5 to 10 kVA.
Why do motors need extra?
A motor, such as in an AC compressor, water pump or fridge, briefly draws 2 to 3 times its running power when it starts.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem