Generator Sizing Calculator
What size generator a set of loads needs — the running watts they draw together, the surge when the largest motor starts, and a recommended rating with a safety margin.
Add up the running watts of everything that will be on at once.
How the generator sizing calculator works
Add up the running watts of everything that will be on at once. Motors — fridges, pumps, air conditioners, power tools — draw several times their running watts for a moment when they start, so the generator must also cover the largest motor’s starting surge on top of everything else already running. Add a margin of about 20–25%, because generators are happiest and last longest below 80% load.
Formula: peak = running total + largest motor × (surge factor − 1); size = peak × (1 + margin)
Worked examples
| Inputs | Recommended generator size | Note |
|---|---|---|
| 3,500 W running with a 1,200 W fridge starting at 3× | 7.5 kW | 5,900 W peak → 7.5 kW |
| Camping: 800 W, no motors | 1 kW | 960 W → 1 kW |
| Workshop: 6,000 W with a 2,200 W compressor at 5× | 18 kW | 14,800 W peak → 18 kW |
FAQFrequently asked questions
What starting surge do motors have?
Roughly 3× running watts for fridges, freezers and pumps, 2–3× for air conditioners, up to 5–6× for compressors and some power tools. The plate may show locked-rotor amps; inverter-driven appliances have little or no surge.
Why size for the largest motor only?
Because motors rarely start at the same instant; the worst case is the biggest one starting while everything else is running. If two large motors can start together, add both surges.
What about kVA versus kW?
Generators are often rated in kVA at 0.8 power factor: kW = kVA × 0.8. A 6.25 kVA set delivers 5 kW. Check which figure the label gives.
Can a generator run too lightly loaded?
Diesel sets run below about 30% load for long periods glaze their bores and wet-stack; petrol sets mind less. Size for the real load, not for every appliance in the house.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- Ofgem — the UK energy regulator
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.