Standby vs Prime Power Calculator
Compare standby and prime power ratings for generators. Calculate derating factors, continuous vs emergency power capacity, and determine the right generator sizing for your application.
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Formulas Used
Rating Conversions (ISO 8528-1):
- Standby Power = Prime Power × 1.10
- Prime Power = Continuous Power × 1.10
- Standby Power = Continuous Power × 1.21
- kVA = kW ÷ Power Factor
Altitude Derating (ISO 3046-1):
- For altitude > 1000 m: Derating Factor = 1 − [(Altitude − 1000) ÷ 300] × 0.03
- No derating below 1000 m
Temperature Derating (ISO 3046-1):
- For temperature > 25°C: Derating Factor = 1 − (T − 25) × 0.01
- No derating at or below 25°C
Combined Derating:
- Combined Factor = Altitude Factor × Temperature Factor
- Derated Power = Nameplate Power × Combined Factor
Prime Overload Capacity:
- Overload Power = Prime Power × 1.10 (for up to 1 hour in every 12 hours)
Effective Output at Load Factor:
- Effective Output = Derated Prime Power × (Load Factor ÷ 100)
Assumptions & References
- ISO 8528-1:2018 — Reciprocating internal combustion engine driven alternating current generating sets: Application, ratings and performance.
- ISO 3046-1:2002 — Reciprocating internal combustion engines: Performance — Standard reference conditions, declarations of power, fuel and lubricating oil consumptions.
- NFPA 110:2022 — Standard for Emergency and Standby Power Systems.
- Standby rating assumes a maximum of 200 hours per year of operation with no sustained overload permitted.
- Prime rating assumes variable load operation with an average load factor not exceeding 70% of the prime rating over any 24-hour period.
- Continuous rating assumes constant load at 100% capacity with no overload permitted.
- Altitude derating applies above 1000 m ASL at a rate of 3% per 300 m (approximately 1% per 100 m).
- Temperature derating applies above 25°C at a rate of 1% per °C above the reference temperature.
- Combined derating is multiplicative (altitude factor × temperature factor).
- A minimum combined derating floor of 50% is applied to prevent unrealistic results at extreme conditions.
- Power factor of 0.8 lagging is the industry standard reference for generator sizing unless otherwise specified.
- The 10% prime overload capacity is permissible for a maximum of 1 hour in every 12 hours of operation.
- Fuel consumption, harmonic distortion, and transient load response are not accounted for in this calculator.