A generator’s “kVA” is not a single fixed number — the same machine carries a standby, a prime, and a continuous rating, and which one applies depends on how hard and how long you run it. ISO 8528, the international standard for generator ratings, defines these for the same hardware. Reading them correctly is the difference between a set that lasts and one that overheats under a load it was never rated to carry.
The three ratings you will see
Standby (ESP — Emergency Standby Power)
Backup only, for 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 only valid for short, emergency use.
Prime (PRP — Prime Running Power)
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 around 70% of the prime rating. A short overload (typically 10% for 1 hour in every 12) is permitted.
Continuous (COP — Continuous Operating Power)
A constant load for unlimited hours, with no overload. This is for true base-load duty — 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 standard a supplier quotes before comparing numbers.
Why one set has three numbers
The engine and alternator are identical; the rating simply reflects the load profile and hours the manufacturer will stand behind. 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 continuous lower still — but always confirm on the specific datasheet, never assume.
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 |
Two follow-on decisions ride on this one. First, the rating interacts with sizing: the load calculation in how to size a diesel generator must be compared against the correct rating, not the biggest number on the page. Second, altitude and ambient heat derate all three ratings equally — 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 procurement error is buying to the standby number and then running the set as prime or continuous power. Running near the standby limit for long hours causes overheating, wet-stacking (unburned fuel fouling the exhaust), and early overhaul. The symptom pattern is familiar to any service engineer: a set that “was fine for months” starts smoking, losing oil pressure, and shutting down on hot afternoons — because it spent those months doing a duty it was never rated for.
The reverse mistake also costs money: specifying continuous rating for a genuine backup application means paying for capacity you will never use.
Reading a datasheet without getting fooled
- Confirm which rating each kVA figure refers to — reputable datasheets label ESP/PRP/COP explicitly.
- Check the reference conditions (altitude, temperature) the rating assumes.
- Confirm frequency (50/60 Hz) and voltage match your market; ratings differ between them.
- If a seller quotes one big number with no rating label, assume it is standby — and ask.
Two questions buyers always ask
“Can I run a standby set as prime power if I keep the load low?” Lightly loading a standby set avoids the overheating failure mode, but it trades it for another: diesel engines dislike running long hours at very light load (glazing, wet-stacking). If the duty is really prime, buy prime — the machine will be loaded correctly by design.
“Is a higher rating just marketing?” No — the ratings carry warranty weight. A manufacturer that labels a set PRP is committing to unlimited-hours operation under the ISO load rules; run it that way and warranty claims stand on solid ground. Run a standby set as prime and the service history will show it.
kVA vs kW — a quick reminder
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, or constant base load — plus your load profile, and 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.