Prime Power (PRP) vs Standby Power (ESP): Which Rating Are You Buying?
By Tommy, Export Sales Manager at Zhiqiang Power · tommy@zhiqiang-power.com · Last reviewed 2026-09-17 against factory load-bank data
Every genset datasheet carries two power figures, and buyers often read only the larger one. The two numbers describe two different contracts between you and the engine: how long it may run, how hard, and what happens when you ask for more.
The short answer
Prime power (PRP) is available for unlimited hours a year at variable load, with 10% overload for one hour in every twelve. Standby power (ESP) is an emergency rating limited to about 200 hours a year, with no overload allowance.
- ›Both definitions come from ISO 8528-1:2018, the same standard every spec table on this site cites.
- ›The standby number is always the bigger one. Comparing one supplier's prime against another's standby flatters the second quote by about 10%.
- ›Running a standby-rated set as your everyday power source shortens its life and usually voids the warranty.
- ›If your site runs more than about 200 hours a year off the generator, you need a prime rating, full stop.
What each rating allows
Prime power (PRP) is the maximum power available for unlimited hours per year, at a variable load, with a 10% overload capability for one hour in every twelve. It is the rating for sites where the generator is part of normal operations: island grids, mining camps, telecom towers with unreliable mains, factories that run the set through peak-tariff hours.
Standby power (ESP) is the maximum power available for emergency backup when the utility fails, limited to about 200 hours per year, with no overload. The average load factor over those hours is expected to be around 70% of the ESP rating. It is the right duty for hospitals, hotels and buildings where the set sits ready and runs rarely.
Why the two quotes you are comparing may not match
A set sold as "100 kW prime" and a set sold as "110 kW" may be the same machine, quoted differently. This is not dishonest in any one market, but it quietly breaks like-for-like comparison. The habit that protects you: always ask which rating the headline number refers to, and compare prime with prime and standby with standby.
Choosing the right duty for your site
Add up your expected annual running hours. Below roughly 200 hours of outage backup, a standby rating is correct and cheaper. Above that, or with daily peak-shaving, specify prime. If your project mixes both duties, say so in the enquiry and we will quote the set against the real duty cycle, with the load-bank test record matching it.
Related reading
- Island / off-grid prime power projects →
- Hospital standby duty guide →
- Technical center (all standards references) →
Power ratings follow ISO 8528-1:2018 (Prime PRP / Standby ESP), and the fuel figures are stated at ISO 3046-1:2002 standard reference conditions. Both are confirmed unit by unit on our load bank, and the test record ships with the set. Standby figures are the engine's factory standby rating from the model spectrum table — never a computed 110% of the rated output. Noise figures are typical open-type reference values, not a guaranteed sound-power level.
Frequently Asked Questions
Can I run a standby generator as my main power source?▼
Not for long. ESP assumes about 200 hours a year of emergency running; beyond that, wear accelerates and most warranties lapse. For regular or continuous running, specify a prime (PRP) rated set of the same power class.
What does the 10% overload rule mean in practice?▼
A prime-rated set may deliver 10% above its PRP figure for one hour in every twelve, useful for motor starts or a short peak shift. It is an allowance inside the prime rating, not a second rating, and standby sets do not get it at all.
Which rating do Zhiqiang quotations use?▼
Both are printed on every datasheet on this site: prime and standby, in kW and kVA, with the ISO 8528-1 reference. Our warranty is benchmarked on the prime baseline, so you know which number you are buying against.
Get a technical quotation for your project
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