About Generator Sizing Calculator
The generator sizing calculator estimates how large an engine-generator set must be to serve a building or facility load. It converts the continuous running load in kW to apparent power in kVA using the power factor, then evaluates the worst-case motor starting scenario, where the largest motor starts against the rest of the running load and draws a large inrush current.
The tool takes the governing of the two scenarios, applies an altitude and temperature derating factor and a spare-capacity margin, and selects the next standard genset rating at or above the requirement. It reports the required kW and kVA, the recommended standard size, and the resulting percent loading so you can confirm the set is neither overloaded on motor start nor wastefully oversized.
How It Works
- Enter the total continuous running load in kW and the overall power factor; the tool computes running kVA = kW / PF.
- Enter the largest motor in HP and its starting method. Inrush apparent power = HP * factor, where the factor is about 6.0 kVA/HP for direct-on-line, 2.0 for star-delta, 3.0 for soft starters, and 1.0 for VFDs.
- The motor starting scenario kVA = (running kVA minus the motor running kVA) + motor inrush kVA. The governing kVA is the larger of the running and starting scenarios.
- A derating factor (for altitude/temperature) and a spare fraction scale the governing kVA up; the calculator then picks the next standard rating and reports percent loading.
Worked Example
A facility has a 150 kW running load at PF 0.8, so running kVA = 150 / 0.8 = 187.5 kVA. The largest motor is 50 HP started direct-on-line at 6.0 kVA/HP, giving an inrush of 300 kVA; the motor running contribution is 50 * 0.746 / 0.8 = 46.6 kVA. The starting scenario = (187.5 - 46.6) + 300 = 440.9 kVA, which governs. Applying a 0.95 derating factor and 10% spare gives 440.9 / 0.95 * 1.10 = 510.5 kVA required, so the next standard size is 600 kVA. Under the 187.5 kVA running load the set is 31.3% loaded.
Formulas
- Running apparent power
kVA = kW / PF- Motor starting scenario
startingKva = (runningKva - HP*0.746/PF) + HP*factor- Required and recommended size
requiredKva = max(runningKva, startingKva) / derate * (1 + spare); recommended = next standard >= requiredKva
Standards & References
- ISO 8528 (reciprocating IC engine driven AC generating sets)
- NFPA 110 (emergency and standby power systems)
- NEC Article 445 / 700 (generators and emergency systems)
Frequently Asked Questions
Why does motor starting often govern the genset size?
A direct-on-line motor draws roughly six times its running kVA for the first few seconds of acceleration. That transient demand, added to the rest of the running load, can far exceed the steady running kVA, so a genset sized only for running load may stall or trip on motor start.
How do soft starters or VFDs change the size?
Reduced-voltage starting cuts the inrush dramatically: star-delta to about a third, soft starters to about half, and VFDs to roughly running current. Using them on large motors can let you choose a much smaller, cheaper genset.
Why apply a derating factor?
Engine and alternator output fall with altitude (thinner air) and high ambient temperature. A derating factor below 1.0 increases the required nameplate rating so the installed set still delivers the needed power at site conditions.
Is the recommended size the same for standby and prime duty?
Not necessarily. Manufacturers publish separate standby (limited hours) and prime (continuous) ratings for the same physical set. This tool sizes the required kVA; confirm against the correct duty rating from the supplier datasheet.