What the calculator does
A generator must do two things: carry the running load continuously, and survive the step when the largest motor starts on top of everything else already running. The calculator works out both requirements in kVA, takes the larger, adds your growth margin, corrects for the thinner, hotter air at your site, and rounds up to the next standard set size. It then checks how loaded the set will be at the normal running load, because a diesel engine that idles at a small fraction of its rating for years is damaged by it.
Formula
Motor input kW = Pmotor ÷ η Motor running kVA = input kW ÷ PFmotor
Step kVA at start = (Prunning − motor input kW) ÷ PF + motor running kVA × kstart
kVA for starting = Step kVA ÷ overload capability
Derating factor = 1 − 3.5 % per 300 m above 900 m − 2 % per 5.5 °C above 40 °C
Recommended prime kVA = max(running, starting) × (1 + margin) ÷ derating factor
where kstart is the starting current as a multiple of running current (about 6 for direct-on-line, 2 for star-delta, 3 for a soft starter, 1.5 for a variable-frequency drive) and the overload capability is the step load the alternator accepts for a few seconds with a tolerable voltage dip, typically 2 to 3 times its continuous kVA. The derating figures are commonly quoted for turbocharged diesel sets; the manufacturer's curve for the engine and alternator governs.
Worked example
An office and workshop in Lahore: 20 kW running load at 0.8 power factor, the largest motor a 5.5 kW compressor started direct-on-line (90 % efficient, 0.85 power factor), a set with 2.5 × starting capability, 25 % margin, sea level, 40 °C.
- Running kVA = 20 ÷ 0.8 = 25.0 kVA.
- Motor input = 5.5 ÷ 0.9 = 6.11 kW; motor running kVA = 6.11 ÷ 0.85 = 7.19 kVA.
- Step at start = (20 − 6.11) ÷ 0.8 + 7.19 × 6 = 17.36 + 43.14 = 60.5 kVA, so the set needs 60.5 ÷ 2.5 = 24.2 kVA of rating to hold its voltage while the compressor starts.
- The running requirement (25.0 kVA) governs. With 25 % margin: 31.3 kVA. Derating at sea level and 40 °C is 1.0.
- Next standard size: 40 kVA prime. At the running load it is 62.5 % loaded, comfortably in the healthy range.
Move the same set to Quetta (1 700 m, 45 °C summers) and the derating factor is 1 − 0.093 − 0.018 = 0.889, so the 31.3 kVA becomes 35.2 kVA; still a 40 kVA set, but with less to spare.
Generator ratings under ISO 8528-1
| Rating | Meaning | Use it for |
|---|---|---|
| ESP — Emergency standby power | Maximum available for the duration of a utility outage; average load ≤ 70 % of ESP, about 200 h a year | backup for load-shedding and outages |
| PRP — Prime rated power | Unlimited hours at variable load; average ≤ 70 % of PRP over 24 h, 10 % overload for 1 h in 12 | sites that run daily or as the main supply |
| LTP — Limited-time running power | Up to 500 h a year at constant load | peak-lopping |
| COP — Continuous power | Constant load, unlimited hours, no overload | base-load and remote sites |
Assumptions and limitations
- One motor starts at a time. If several large motors can start together, add their starting kVA, or sequence them with time delays, which is the cheaper answer.
- Starting multipliers are typical. Locked-rotor current ranges from 5 to 8 times full-load for direct-on-line starts depending on the motor design; use the nameplate code or datasheet where it matters.
- Voltage dip is not calculated. The overload capability input stands in for the alternator's transient response. For sensitive loads or large motors, ask the supplier for a motor-starting calculation with the actual alternator data.
- Non-linear loads need more. UPS rectifiers, variable-frequency drives and LED drivers distort the current; a common rule is to size the alternator at 1.5 to 2 times the kVA of such loads, or specify a low-impedance alternator.
- Fuel, noise, emissions and switchgear are separate. The result is a rating, not a specification.
Frequently asked questions
Why is the set rated in kVA but my loads are in kW?
The alternator is limited by current, so its rating is apparent power. Its kW capability is the kVA times the rated power factor, 0.8 by convention: a 40 kVA set delivers 32 kW. The engine behind it is sized for that 32 kW.
What happens if the set is too big?
A diesel engine run below about 30 % of its rating for long periods does not reach working temperature, unburnt fuel and carbon collect in the exhaust (wet stacking), the injectors foul and service life falls. It also costs more to buy and burns more fuel per kWh. If the load is small now and will grow later, choose a set for now and plan the second one.
Can I avoid oversizing for one motor?
Usually. A soft starter or variable-frequency drive on the largest motor cuts its starting step by half or more; sequencing starts so that the largest motor starts first on an unloaded set helps too. Both are cheaper than the next generator size.
Does natural gas change the sizing?
The electrical arithmetic is the same. Gas engines derate more steeply with altitude and temperature and respond more slowly to step loads, so the overload capability is usually lower; take the figures from the specific set.
References
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets — Part 1: Application, ratings and performance — COP, PRP, LTP and ESP ratings
- ISO 8528-5:2022, Part 5: Generating sets — performance classes, transient voltage and frequency limits on step loads
- ISO 3046-1:2002, Reciprocating internal combustion engines — Performance — Part 1: Declarations of power, fuel and lubricating oil consumptions, and test methods — reference conditions and power adjustment for site conditions
- IEC 60034-12:2016, Rotating electrical machines — Part 12: Starting performance of single-speed three-phase cage induction motors — locked-rotor apparent power classes