Sizing a generator means matching two limits to your load. The alternator is limited by apparent power (kVA) and the engine by real power (kW), linked by power factor. You convert the running load to kVA, add a spare margin for growth and engine life, then check the hardest moment — usually starting the largest motor, which briefly demands several times its running current. The set must ride through that surge without an excessive voltage or frequency dip.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: ISO 8528 (generating sets) and major genset sizing guides.
Engineering notice. This is a first-pass estimate. Final selection must use the manufacturer's transient and block-load capability, harmonic and non-linear load data, altitude and temperature de-rating, and be confirmed by a qualified engineer. See our disclaimer.
How generator sizing works
Generators are rated at 0.8 power factor, so a genset's kW output is 0.8 of its kVA. A spare margin of 20–25% is typical for life and growth. If a large motor starts direct-on-line, the brief surge can be the deciding factor.
Worked example — workshop with a big motor
Scenario: Total running load 120 kW at 0.8 PF, with a 30 kW motor started by soft starter, 25% margin.
The steady sized figure (188 kVA) and the soft-start allowance (about 195 kVA) are close, so the next standard size is 200 kVA (160 kW). If that motor were started direct-on-line, the surge would push the requirement higher — which is exactly why a soft starter or VFD lets you fit a smaller, cheaper generator.
Frequently Asked Questions
Total the running kW, divide by power factor for kVA, add a 20–25% margin, and check it can start your largest motor. Then pick the next standard size. 80 kW at 0.8 PF = 100 kVA; +25% = 125 kVA.
kVA is apparent power (alternator limit), kW is real power (engine limit): kW = kVA × PF. Generators are rated at 0.8 PF, so a 100 kVA set delivers 80 kW.
A 20–25% margin covers load growth, extends engine life by keeping it off its maximum continuous rating, and gives headroom for transient and non-linear loads.
A direct-on-line motor draws several times its running current at start, demanding a burst of kVA and dipping voltage/frequency. The set must start the biggest motor without that dip tripping other loads — often the governing factor.
At 0.8 PF, 100 kW = 125 kVA; +25% margin ≈ 156 kVA, so a 160 kVA standard genset — more if a large motor starts on it. Confirm with the maker's block-load curves.