Power & UPS Tool
Size a standby, prime or continuous diesel generator to ISO 8528 planning practice — from connected load and duty class to altitude/temperature derating, required kVA and fuel autonomy.
Quick Answer
Enter connected load, power factor and ISO 8528 duty class (ESP/PRP/COP), then site altitude and ambient temperature. The calculator applies duty-class loading and site derating to recommend the next standard genset size.
Genset Sizing
Enter connected load and site conditions. Required rating is estimated automatically for early planning.
Required kVA ≈ (load ÷ PF) ÷ duty-class loading ÷ (1 − site derate). Output current assumes 400 V three-phase.
This is a planning-level estimate. Manufacturer sizing accounts for motor starting current, voltage-dip limits and site-specific derating curves.
Formula / Method
The calculator converts connected load to apparent power (kVA), divides by the ISO 8528 duty-class loading target (ESP 90%, PRP 80%, COP 70%) to keep headroom, then divides by the site derating factor for altitude and ambient temperature above reference conditions. The result is matched to the next standard genset rating.
Required kVA ≈ (load ÷ PF) ÷ duty-class loading ÷ (1 − site derate)
Inputs
Use connected load and its unit (kW or kVA), power factor, ISO 8528 duty class, site altitude and ambient temperature. Fuel tank volume, specific consumption and largest step load are optional.
Uses
Useful for early standby/prime power planning, data centre and industrial backup-power sizing checks, site altitude/temperature derating awareness, fuel autonomy planning and ATS/genset coordination discussions.
FAQ
ESP (Emergency Standby Power) is for standby use with limited annual hours and no sustained overload headroom — it uses the highest loading target (90%). PRP (Prime Power) supplies variable, unlimited-time loads with a defined overload allowance, so it targets 80% loading for margin. COP (Continuous Power) runs at a constant, unvarying load with no overload capability, so it targets the most conservative 70% loading. Lower loading targets mean a larger genset is recommended for the same connected load.
No. This is a planning-level estimate. Motor and step-load transients, voltage-dip limits, nonlinear loads, harmonics, fuel and emissions requirements, and automatic transfer switching (ATS) all need manufacturer sizing software and the relevant ISO 8528-5 performance class (G1–G4) for the connected loads.
Engine and alternator output derate above roughly 1000 m altitude and above a reference ambient temperature (commonly 25 °C), per ISO 3046 planning guidance — air density drops with altitude and cooling margin drops as ambient temperature rises, so the same genset delivers less usable power on site than its nameplate rating at standard reference conditions.
Learn More
Project
ITCOREOPS can support genset sizing coordination, ATS planning, fuel autonomy planning and infrastructure documentation.
Disclaimer
Planning-level sizing, not manufacturer selection or a load-schedule substitute. Motor/step-load transients and voltage-dip limits, nonlinear loads, fuel & emissions, transfer switching (ATS), and N+1/2N redundancy (a separate tool) are out of scope; verify with the genset manufacturer and a qualified engineer.
Workflow
Use this calculator before requesting manufacturer quotes, planning fuel storage, coordinating ATS transfer or documenting backup-power requirements.
Support
For genset sizing, ATS coordination, fuel planning and operational infrastructure support, visit ITCOREOPS.
Feedback
Report display problems, calculation issues or suggestions for improving this calculator.
Worked example
Inputs: 200 kW connected load at power factor 0.8, ISO 8528 duty class PRP (Prime), site altitude 500 m, ambient temperature 30 °C, fuel tank 1000 L at 0.25 L/kWh specific consumption.
Result: base rating 250.0 kVA ÷ 0.80 (PRP loading) ÷ 0.98 (2.0% temperature derate) ≈ 318.9 kVA required → next standard size 350 kVA (PRP); output current ≈ 460.3 A at 400 V three-phase; fuel autonomy ≈ 20.0 h from a 1000 L tank.
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