A generator’s kVA is not one fixed number. The same machine has a standby, a prime and a continuous rating, and the one that applies depends on how hard and how long you run it. ISO 8528, the international standard for generator ratings, defines all three for the same hardware. Read them correctly and the set lasts. Read them wrongly and it overheats under a load it was never rated to carry.
The three ratings you will see
Standby (ESP: Emergency Standby Power)
For backup only, when the mains fails. It allows a variable load with no sustained overload, and a limited number of running hours per year, commonly up to around 200 hours. It is the highest number on the datasheet, but it is valid only for short emergency use.
Prime (PRP: Prime Running Power)
For the main power source where there is no grid. It allows unlimited running hours at a variable load, provided the average load over 24 hours stays at about 70% of the prime rating. A short overload is permitted, typically 10% for 1 hour in every 12.
Continuous (COP: Continuous Operating Power)
For a constant load for unlimited hours, with no overload. This rating is for true base-load duty, such as feeding a steady industrial load 24/7 or running in parallel with the grid.
A fourth rating, LTP (Limited-Time Prime), allows a fixed load for a limited number of hours per year. Check which rating a supplier quotes before you compare numbers.
Why one set has three numbers
The engine and alternator are the same. Each rating reflects the load profile and running hours that the manufacturer will guarantee. The numbers always rank in the same order:
Standby > Prime > Continuous
As a rough guide, the prime rating is often near 90% of the standby figure, and the continuous rating is lower still. Always confirm the figures on the specific datasheet.
How to choose
| Your situation | Use this rating |
|---|---|
| Backup for utility failures | Standby |
| Off-grid primary supply, load varies | Prime |
| 24/7 constant base load | Continuous |
This choice affects two other decisions. First, sizing: compare the load calculation in how to size a diesel generator with the correct rating, not with the largest number on the page. Second, altitude and ambient heat reduce all three ratings by the same percentage. A prime rating quoted at sea level is not your prime rating at 3,000 m (see diesel derating at altitude).
The expensive mistake
The most common purchasing error is to buy by the standby number and then run the set as prime or continuous power. Long hours near the standby limit cause overheating, faster oil breakdown and an early overhaul. Service engineers see the same pattern often. The set runs for months, then starts to smoke, lose oil pressure and shut down on hot afternoons, because it has been doing a duty it was never rated for.
The opposite mistake also costs money. Specifying a continuous rating for a real backup application means paying for capacity you will never use.
How to read a datasheet
- Confirm which rating each kVA figure refers to. Reliable datasheets label ESP, PRP and COP explicitly.
- Check the reference conditions, such as altitude and temperature, that the rating assumes.
- Confirm that the frequency (50/60 Hz) and voltage match your market; the ratings differ between them.
- If a seller quotes one large number with no rating label, assume it is the standby rating, and ask.
Two questions buyers always ask
“Can I run a standby set as prime power if I keep the load low?” A light load avoids overheating, but it causes a different problem. Diesel engines wear when they run long hours at very light load, through cylinder glazing and wet-stacking. If the duty is prime, buy a prime-rated set, so that the engine runs at the load it was designed for.
“Is a higher rating just marketing?” No. The ratings are tied to the warranty. A manufacturer that labels a set PRP commits to unlimited running hours under the ISO load rules. If you run the set that way, warranty claims are on solid ground. If you run a standby set as prime, the service history will show it.
kVA and kW
Generators are rated in kVA, but loads are usually given in kW:
kW = kVA × power factor (typically ≈ 0.8)
So a 100 kVA prime-rated set delivers roughly 80 kW of prime power.
Tell us your duty (backup, off-grid primary supply or constant base load) and your load profile. Our engineers will match the right HTX set from the diesel generator range at the right rating. Request a quote or message us on WhatsApp; we reply within 24 business hours.