Generator Size Calculator
Size a standby generator for running and starting loads.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Generator Size Calculator does
Generator sizing must cover the total running load plus the surge when the largest motor starts. Undersizing causes voltage dips and stalling; heavy oversizing wastes fuel and damages diesel engines through light-load running.
Formula
Peak demand = Running load + Largest motor starting surgeRequired = max(Running × (1 + Headroom), Peak demand) ÷ DeratingkVA = Required watts ÷ Power factor ÷ 1,000
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Total running watts | W | Yes | Accepts more than 0. |
| Largest motor starting watts | W | Yes | Accepts 0 or more. |
| Spare capacity | % | Yes | — |
| Power factor | number | Yes | Accepts 0.1 or more, up to 1. |
| Altitude and temperature derating | % | Optional | Generators lose output at altitude and in heat. |
| Fuel use at full load | L/hour | Optional | Accepts 0 or more. |
| Expected run hours | number | Optional | Accepts 0 or more. |
| Fuel price per litre | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Currency.
- Enter Total running watts, Largest motor starting watts, Spare capacity and Power factor.
- Optionally add Altitude and temperature derating, Fuel use at full load and Expected run hours.
- Select Calculate.
Worked example
A 4,000 W running load with a 6,000 W motor surge, 20% spare, 0.8 power factor.
- Running
- 4000 W
- Starting
- 6000 W
- Headroom
- 20
- PF
- 0.8
Peak demand 10,000 W, required 10 kW → 12.5 kVA needed, so a 15 kVA set.
Frequently asked questions
Why not just buy the biggest generator affordable?
Diesel generators running below about 40% load suffer wet stacking — unburnt fuel fouling the exhaust and shortening engine life.
Method and sources
Method. Peak demand from running load plus the starting surge of the largest motor, converted to kVA at the power factor entered and derated for altitude and temperature.
Applies to. None applied — generator installation and connection are governed by national regulations
Assumptions
- Only one large motor starts at a time, which is the usual but not guaranteed case.
Limitations
- Starting multiples are conventions by motor type, and a specific machine can differ substantially — its own data plate governs.
- Derating for altitude and temperature follows manufacturer curves that vary by engine; the factors here are indicative.
- Oversizing carries its own problem: a diesel generator run at very light load wet-stacks and degrades, so the largest safe answer is not the best one.
Sources
- The generator manufacturer's derating curves, and the national regulations governing installation and grid connection — Varies by manufacturer and country. The derating factors and every aspect of installation, including transfer switching and earthing.