Generator size calculator
Add up the loads that run together plus the start-up surge of the biggest motor to pick a generator that will not trip.
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How to use it
- Enter total running load, starting watts of largest item, running watts of that item, safety margin and power factor.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Peak = total running watts + (largest starting watts minus its running watts). Recommended = peak × (1 + margin). kVA = kW ÷ power factor.
Example
Inputs: Total running load = 3,000 W, Starting watts of largest item = 2,100 W, Running watts of that item = 700 W, Safety margin = 20 %, Power factor = 0.8. Result: Recommended generator power: 5.28 kW; In kVA: 6.6 kVA; Starting peak: 4,400 W.
Limits and caveats
- Motor loads (AC, fridge, pump) usually start at 2 to 3 times their running watts; the maker figure is more accurate.
- 0.8 is the usual power factor in generator ratings; change it if yours is rated differently.
- It assumes only one item starts at a time.
Questions about this tool
What is the difference between kW and kVA?
kVA is apparent power and kW is real power, equal to kVA times the power factor; 10 kVA at 0.8 gives 8 kW.
Why add starting watts?
Motors draw a strong surge for a few seconds at start; if the generator cannot supply it, it trips or the voltage sags.
What safety margin should I use?
Many people allow 20% to 25% so the generator is not run at its limit all the time.