Every generator datasheet shows two or three power figures. Mixing them up is the most common — and most expensive — mistake in generator buying. Here is what each rating means under ISO 8528-1, and which one your project needs.
Emergency standby power (ESP)
The highest number on the datasheet. It is the power the set can deliver when the mains has failed, for a limited number of hours per year (typically up to around 200), with an average load over 24 hours of no more than about 70% of the standby rating. There is no overload capability above it.
Use standby when the generator is a backup for a reliable grid: offices, towers, villas, schools, hotels in the UAE.
Prime power (PRP)
Prime is the power the set can deliver for an unlimited number of hours per year with a varying load, again with the 24-hour average kept to about 70% of the prime rating. Most manufacturers allow a 10% overload for one hour in every twelve on prime.
Prime is typically about 10% lower than standby for the same machine. A set sold as "500 kVA standby" is roughly a 450 kVA prime set.
Use prime when the generator is the power supply — or runs many hours a day:
- construction sites and camps with no grid connection
- farms, quarries and remote sites
- markets with long daily outages, such as Baghdad or Basra
Continuous power (COP)
A lower rating again, for a constant 100% load running without limit — base-load power plants and some industrial processes. Data centres often ask for a data-centre continuous rating; tell us if your consultant specifies one.
Why it matters when you compare quotes
If supplier A quotes 500 kVA standby and supplier B quotes 500 kVA prime, supplier B is offering a bigger machine — so it will cost more. Always ask which rating a price refers to, and compare like with like. It is one of the nine factors that decide generator price.
Heat changes the numbers too
Ratings are given at standard reference conditions. At 45–50 °C the engine and alternator must be derated, so in the Gulf we check the site rating at your real ambient temperature, not just the catalogue figure.
Quick rule
- Grid is reliable, generator is backup → size on standby.
- Generator runs daily or is the only supply → size on prime, and allow for heat.
Our generator size pages show both the standby kVA and the matching prime figure for every size from 10 to 2500 kVA. Not sure which applies? Send us your run hours and load list on WhatsApp.
