Power & UPS
Generator Sizing Reference
ISO 8528 duty classes, altitude/temperature site derating, the standard kVA rating series, ISO 8528-5 performance classes and typical fuel consumption. Use alongside the Generator Sizing calculator.
Last updated: July 2026
Terms
- UPSUninterruptible Power Supply
- A battery backup that keeps equipment running for a short time if mains power cuts out, giving generators time to start or equipment time to shut down safely.
- kVAKilovolt-Ampere
- A unit of electrical power, similar to kW but measured slightly differently — it includes power that's drawn but not fully put to use. Equipment ratings often use kVA because it's the more cautious, safer number to size for.
ISO 8528 Duty Classes
ISO 8528 defines four power ratings for generating sets. Planning-level sizing typically uses ESP, PRP or COP — the loading target directly drives the recommended genset size for a given connected load.
| Class | Full name | Loading target | Typical annual hours | Overload allowance | Typical use |
|---|---|---|---|---|---|
| ESP | Emergency Standby Power | 90 % | ≤ 200 h/year | None above rating | Standby backup during utility outage |
| PRP | Prime Power | 80 % | Unlimited | 10 % for 1 h in 12 h | Variable load, no grid connection or grid-parallel |
| COP | Continuous Power | 70 % | Unlimited | None above rating | Constant, unvarying base load |
| LTP | Limited-Time Running Power | — | ≤ 500 h/year | None above rating | Peak shaving, temporary supply |
Site Derating — Altitude
Engine and alternator output derate above roughly 1000 m due to reduced air density. Planning-level approximation: ~1 % per 100 m above 1000 m.
| Altitude | Derate (planning estimate) |
|---|---|
| 0 – 1000 m | 0.0 % |
| 1500 m | 5.0 % |
| 2000 m | 10.0 % |
| 2500 m | 15.0 % |
| 3000 m | 20.0 % |
Site Derating — Ambient Temperature
Cooling margin drops as ambient temperature rises above the reference condition (25 °C). Planning-level approximation: ~2 % per 5 °C above 25 °C.
| Ambient temperature | Derate (planning estimate) |
|---|---|
| ≤ 25 °C | 0.0 % |
| 30 °C | 2.0 % |
| 35 °C | 4.0 % |
| 40 °C | 6.0 % |
| 45 °C | 8.0 % |
| 50 °C | 10.0 % |
Combined derate is capped at 60 % in the calculator. Example: 2000 m and 40 °C together derate 16 % (10 % altitude + 6 % temperature).
Worked Example — 200 kW Prime Power Load
Connected load: 200 kW at power factor 0.8. Duty class: PRP. Site: 500 m altitude, 30 °C ambient.
Step 1 — Base rating: 200 kW ÷ 0.8 PF = 250.0 kVA
Step 2 — Site derate: 0 % altitude (500 m < 1000 m) + 2.0 % temperature (30 °C) = 2.0 %
Step 3 — Duty-class loading: PRP targets 80 % loading
Step 4 — Required rating: 250.0 ÷ 0.80 ÷ (1 − 0.02) = 318.9 kVA
Step 5 — Next standard size: 350 kVA (PRP)
Step 6 — Output current at 400 V 3~: 318.9 kVA × 1000 ÷ (√3 × 400 V) = 460.3 A
Standard kVA Rating Series
Common catalogue ratings for diesel gensets, with the equivalent kW at 0.8 power factor and output current at 400 V three-phase.
| kVA | kW (0.8 PF) | Current at 400 V 3~ |
|---|---|---|
| 20 | 16 | 28.9 A |
| 40 | 32 | 57.7 A |
| 80 | 64 | 115.5 A |
| 125 | 100 | 180.4 A |
| 200 | 160 | 288.7 A |
| 350 | 280 | 505.2 A |
| 400 | 320 | 577.4 A |
| 630 | 504 | 909.3 A |
| 1000 | 800 | 1443.4 A |
| 2000 | 1600 | 2886.8 A |
ISO 8528-5 Performance Classes
Performance class describes the genset's voltage/frequency response to load steps — critical for IT and other sensitive loads, separate from the ESP/PRP/COP duty rating.
| Class | Voltage dip on load step | Recovery time | Typical application |
|---|---|---|---|
| G1 | Not tightly controlled | Not specified | General purpose, non-sensitive loads |
| G2 | ≤ 15 – 20 % | Few seconds | General commercial/industrial |
| G3 | ≤ 10 – 15 % | ≤ 1.5 s | IT and data centre standby — most common target |
| G4 | ≤ 10 % | ≤ 0.5 s | Sensitive electronic/medical loads |
Typical Diesel Fuel Consumption
Approximate specific consumption for a diesel genset by load point. Actual figures vary by engine model — always check the manufacturer's fuel curve for final planning.
| Load point | Typical specific consumption | Notes |
|---|---|---|
| 25 % load | 0.30 – 0.40 L/kWh | Inefficient — avoid sustained light-load running |
| 50 % load | 0.27 – 0.32 L/kWh | Improving efficiency |
| 75 % load | 0.24 – 0.28 L/kWh | Near the sweet spot for most diesel engines |
| 100 % load | 0.23 – 0.27 L/kWh | Best specific efficiency, no overload margin |
Frequently Asked Questions
Why is PRP loading 80 % and not 100 %?
Prime Power allows unlimited running hours with a variable load, so ISO 8528 reserves headroom (a 10 % overload allowance for 1 hour in 12) for load transients without exceeding the engine's continuous rating. Sizing to 80 % of nameplate at the expected peak load keeps the set within its variable-load duty envelope.
Do I need G3 performance class for a data centre?
Most IT standby applications target G3 (≤10–15 % voltage dip, recovery within 1.5 s) so UPS systems and sensitive electronics ride through the transfer without tripping. G4 is reserved for especially sensitive medical or precision-electronic loads; G1/G2 are acceptable for general commercial loads without strict voltage-dip requirements.
Is altitude derating the same for all genset models?
No — the ~1 %/100 m figure here is a planning-level approximation per ISO 3046 guidance. Turbocharged and electronically-governed engines often derate less aggressively than naturally-aspirated ones. Always confirm with the manufacturer's altitude/temperature correction curve before finalising a purchase order.
How much fuel storage do I need for a given autonomy?
Multiply the connected load (kW) by the specific consumption at that load point (L/kWh) to get litres per hour, then multiply by the required autonomy in hours. Add margin for tank heel (unusable fuel) and planned refuelling logistics — see the Generator Sizing calculator's fuel-autonomy output for a live figure.