Generator Sizing Calculator
Work out the generator size (in kVA) you need for a given running load in kW, allowing for the load's power factor and a spare-capacity margin, then round up to the next standard off-the-shelf genset rating.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter your total running load P in kilowatts (kW) — the sum of everything the generator has to run at once.
- Set the load power factor (default 0.8 for a mixed motor/lighting load) and the spare-capacity margin as a percentage (default 25%).
- Read the required kVA, the recommended standard genset rating, and how much real-power (kW) headroom that rating leaves.
How it works
Generators are rated in apparent power (kVA) while loads are quoted in real power (kW), so the load is first converted to apparent power S = P / pf. A spare-capacity factor is then applied for motor-starting surges and future growth: required kVA = (P / pf) × (1 + spare/100). The result is rounded up to the next rating in a common off-the-shelf range (8, 10, 13 … 550 kVA), and the leftover real-power headroom (rating × pf − P) is reported.
Worked example
Worked example. A site has a 40 kW running load at a power factor of 0.8 and wants 25% spare. Apparent power S = 40 / 0.8 = 50 kVA; required = 50 × 1.25 = 62.5 kVA. The next standard rating up is 66 kVA, which delivers 66 × 0.8 = 52.8 kW — about 12.8 kW of real-power headroom above the 40 kW load.
Common mistakes
- Sizing the set in kW instead of kVA — a low power factor means the alternator sees more kVA than the kW load suggests.
- Ignoring motor-starting (in-rush) current, which can be 6–8× run current and may need a bigger set or a soft starter even when steady-state kVA looks fine.
- Using a power factor of 1 for a motor-heavy load; real inductive loads run nearer 0.8, which raises the required kVA.
Frequently asked questions
Should I use standby or prime power ratings?
This tool sizes to the steady running (prime) load plus a margin. Genset data sheets quote both a lower continuous/prime rating and a higher short-duration standby rating — always match the manufacturer's rating type to how the set will actually be used.
Why add spare capacity instead of picking the exact required kVA?
Headroom covers motor in-rush, future load growth, ambient/altitude derating and keeps the set off its thermal limit for better fuel efficiency and engine life. 25% is a common default, but adjust it to your load type.
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