Standby is for emergencies only — limited hours per year, no sustained overload. Prime is for unlimited running hours where there is no grid, provided your average load stays around 70% of the rating. Same machine, two numbers: the standby figure is always the larger one, and it is the one most buyers accidentally purchase against. Getting this wrong doesn’t show up on delivery day — it shows up as an early overhaul.
The two ratings side by side
| Prime (PRP) | Standby (ESP) | |
|---|---|---|
| Purpose | Main power source, no grid available | Backup when the mains fails |
| Running hours | Unlimited | Limited — commonly ~200 h/year |
| Load profile | Variable, ~70% average over 24 h | Variable, no sustained overload |
| Overload allowance | ~10% for 1 hour in every 12 | None |
| Relative size | Lower number | Higher number (typically ~10% above prime) |
| Typical use | Mine, farm, off-grid site, remote works | Shop, clinic, office, residential backup |
A third rating, continuous (COP), sits below prime: a constant load, unlimited hours, no overload allowance at all — for true 24/7 base load. The full definitions under ISO 8528 are in generator power ratings explained.
Why the same machine carries two numbers
The engine and alternator don’t change. What changes is what the manufacturer will stand behind: how many hours, at what average load, with how much overload headroom. A set sold as “15 kVA standby” might be “13.6 kVA prime” — the metal is identical; the promise is not.
That is why the standby figure is the one on the poster. It is the biggest honest number the machine can be labelled with, and it is valid only for the smallest job.
How to choose, in one question
Is this machine the main source of power, or the fallback?
- Main source (no grid, or grid unavailable for long stretches) → buy against the prime rating.
- Fallback (utility exists, generator covers outages) → buy against the standby rating.
- Constant industrial base load, 24/7 → ask about the continuous rating.
If your answer is “mostly backup, but it also runs the site during the dry season” — that is prime duty. When in doubt, size against prime; the cost difference is small compared with an engine overhaul.
The expensive mistake, in detail
The classic error: buy a “20 kVA” set advertised at its standby figure, then run it 10 hours a day as the site’s main supply. What follows is predictable:
- The engine spends its life near a limit intended for occasional emergencies.
- Cylinder temperatures and load factors stay outside the healthy band, so wet-stacking begins — unburned fuel fouling the exhaust and injectors (the same mechanism described in fuel consumption per hour).
- Power drops, smoke increases, oil consumption rises.
- Overhaul arrives years earlier than it should — and warranty conversations get difficult, because the service history shows a standby machine doing prime work.
The reverse mistake is cheaper but still real: specifying continuous rating for a genuine backup application means paying for capacity you will never use.
What this means for sizing
Rating and sizing are two different steps, and they must be done in the right order:
- Total your real loads and the largest motor start — the method is in how to size a diesel generator.
- Correct for altitude and temperature (a highland site loses 15–20%; see altitude derating).
- Then compare that corrected figure against the correct rating for your duty — not against the biggest number on the datasheet.
The class-by-class picture, with the same warning about which rating a chart is quoting, is in the kVA size chart.
Three questions that settle it with any supplier
- Is the kVA you quoted prime or standby?
- What is the other figure for the same machine?
- At what reference conditions (altitude, temperature) — and what does it become at my site?
A supplier who answers all three quickly is one you can plan with. HTX states both prime and standby figures on its generator set product pages — see the diesel generator range, or the emergency power page if your duty is genuinely backup.
Not sure which rating your duty falls under? Tell us how many hours a day the set will run and whether the grid exists — or message us on WhatsApp — and our engineers will confirm the rating and the right model within 24 business hours.