Prime power and standby power are two different machines at the same kVA figure. A prime-rated set carries a variable load for an unlimited number of hours and is allowed a 10 % overload for one hour in twelve; a standby-rated set is declared for the duration of a mains failure, carries no overload allowance, and is normally capped at a few hundred running hours a year. A 500 kVA standby set and a 500 kVA prime set are not interchangeable, and the prime one is the more expensive machine.
The three ratings you will see on a quotation
| Rating | Typical name | Duty | Overload allowance |
|---|---|---|---|
| ESP | Emergency standby power | Back-up during a mains failure, variable load | None |
| PRP | Prime power | Variable load, unlimited hours | 10 % for 1 hour in 12 |
| COP | Continuous power | Constant load, unlimited hours | None |
Some catalogues add a fourth line, LTP (limited-time running power), for the occasional long outage on a standby set. If a supplier quotes LTP, treat it as a rating for a defined number of hours a year rather than as a continuous rating, and ask which line the price refers to.
What the difference means in kW
The kVA figure is the alternator; the kW figure is the engine. At 0.8 power factor the arithmetic is simple, and it is where rating confusion costs money:
| Rating at 500 kVA | Engine rating | Allowed for one hour in twelve |
|---|---|---|
| ESP (standby) | 400 kW | No overload – 400 kW is the ceiling |
| PRP (prime) | 400 kW | 440 kW |
| COP (continuous) | 400 kW | No overload – the load must be steady |
The numbers move with the rating band as well: a 500 kVA set and a 1000 kVA set differ in far more than the alternator, because the cooling package, the fuel system and the alternator temperature rise class all follow the duty.
Choosing between them
- Take standby when the mains is reliable and the set only covers an outage: offices, hotels, retail, a hospital block with a second independent feed.
- Take prime when the set is the supply: construction sites, remote camps, oil and gas locations, and anything running eight to twenty-four hours a day for weeks.
- Take continuous when the load does not vary and the set never stops – some process and telecom applications. A variable load on a continuous-rated set is a mis-application, not a saving.
The practical consequence: a standby set used as a prime set is outside the conditions it was declared for. The usual symptoms are rising oil consumption, exhaust smoke and a warranty discussion. The cost of moving up to the prime rating is far smaller than the cost of a failed set on a site with no mains.
Worked example: a construction site, not a hotel
- Site load: a batching plant, two tower cranes and site accommodation, running 280 kW at 0.8 PF = 350 kVA, fourteen hours a day, six days a week.
- Annual expectation: about 4000 running hours a year. That is far outside any standby rating.
- Rating needed: PRP, or COP if the load is steady, at 350 kVA plus 10 to 20 % margin – so a 400 kVA prime or a 500 kVA prime set, depending on the crane starting method.
- The same site with a standby-rated 400 kVA set would be running at its ceiling for most of the day, with no allowance for the crane starts.
Compare that with a hotel on the same 350 kVA of load: the mains carries it for 8700 hours a year, the set covers the rest, and an ESP-rated set is the correct – and cheaper – purchase. The construction site and hotel pages describe the two profiles in more detail.
Conditions that change the answer
- Annual hours. A standby set asked to run 2000 hours a year is a mis-rated set, whatever the kVA says. Write the expected annual hours on the enquiry.
- Ambient. Both ratings are declared at reference conditions; a set that is already de-rated for 50 C has less headroom above its rating, not more.
- Load shape. A prime rating assumes a varying load. A constant load at 100 % of the prime rating is a continuous application in practice.
- Redundancy. Two sets at N+1 are not sized like one set – each of them has to carry the critical load on its own, and the duty changes with the number of running hours per set.
Questions
Can I run a standby generator 24 hours a day?
Not within its declared rating. Standby (ESP) ratings carry no overload allowance and are intended for the duration of a mains failure, with an annual usage cap that manufacturers commonly put in the low hundreds of hours. For continuous operation you specify PRP or COP, which means a heavier alternator and a bigger cooling package at the same kVA – typically a step up in price, not a step down.
What does a 10 % overload allowance mean in kW on a 500 kVA prime set?
400 kW × 1.10 = 440 kW, for one hour in every twelve. On a 500 kVA standby set the same calculation is not available at all: 400 kW is the ceiling. If the load list has a 430 kW peak on a 500 kVA requirement, the correct answer is a prime-rated set or the next size up, not a standby set run into its overload.
Why is a prime-rated set more expensive than a standby-rated one at the same kVA?
Because the components are declared for continuous duty. The alternator is wound for a lower temperature rise class, the radiator and fan are sized for sustained full-load heat rejection, and the fuel system is expected to run for thousands of hours a year. Ask for both prices on the same datasheet and the difference is visible in the alternator class, not in the kVA number.
Which rating should an enquiry in the Gulf use?
Write the duty, not just the rating name: “standby, mains failure back-up, expected 200 hours a year” or “prime, 4000 hours a year, site with no mains”. Add the ambient temperature and the altitude. With those three lines the rating is unambiguous, and the quotation can be compared with any other supplier’s on equal terms. Send it through the request a quote form.
Where the rating is written down
The rating belongs in three places: on the enquiry, on the quotation and on the datasheet that comes with the set. If a quotation for a diesel generator set shows a kVA figure without naming the rating, ask which one it is before comparing it with anything else – two 500 kVA quotations can be two different machines. The sizing method itself is in how to size a diesel generator.
Tell us the duty and the annual hours
Standby or prime, the expected running hours and the site conditions are enough for us to quote the right rating the first time.
Written and maintained by the KVA Grid sales engineering desk. Rating definitions follow ISO 8528-1 as declared by the manufacturer; annual-hour figures are usage guidance, not a standard limit. Reviewed 26 September 2026.