The three ratings that decide what a generator is allowed to do — and the distinction the used market turns on.
Every industrial generator carries an ISO 8528 duty rating — standby, prime, or continuous — and the rating decides what the machine is legally and mechanically allowed to do. Buyers who ignore it buy a standby set for prime duty and kill it in two years. Sellers who state it correctly sell faster. This is the distinction the whole used market turns on.
It’s on the nameplate — photograph it when you sell. Sell your generator →
| Rating | ISO 8528 | Allowed use | Overload | Typical hours/yr |
|---|---|---|---|---|
| Standby (ESP) | Emergency Standby Power | Backup during utility outage only | None | ~200 |
| Prime (PRP) | Prime Rated Power | Primary power, variable load, unlimited hours | 10%, 1 hr in 12 | Unlimited |
| Continuous (COP) | Continuous Operating Power | Constant load, unlimited hours | None | Unlimited (8,760) |
The same physical machine often carries three different numbers — e.g. 100 kW standby / 90 kW prime / 80 kW continuous. The derate between them is the manufacturer telling you how hard each duty cycle is on the machine. A listing that says “100 kW” without the rating is hiding the only number that matters for your application.
For outages — hospitals, data centers, commercial buildings with utility service. Runs occasionally, must start fast and carry full load immediately. Never run a standby set as primary power: no overload allowance and ~200 hrs/year design life means prime duty destroys it. Most used sets on the market are standby — low hours, clean, and exactly right for backup.
For off-grid sites, construction, oil & gas, events — anywhere the generator is the utility. Unlimited hours at varying load, with a 10% overload allowance one hour in twelve. Prime sets are built heavier (cooling, bearings, alternator) and priced accordingly. If your application runs daily, you need prime — a standby set will not survive it.
Constant load, 24/7 — cogeneration, peak shaving, remote facilities. No overload, ever: the rating assumes the machine never gets a break from full load. The most conservative rating — and the one that commands the most respect from buyers who know, because a continuous-rated set at its rating will run for decades.
When buying used: match the rating to the duty — standby for backup, prime for daily use. A low-hour standby set is the best value in backup power; a high-hour prime set is a known quantity with maintenance records (or a gamble without them). When selling: state all three ratings from the nameplate. “100 kW standby / 90 kW prime” sells faster and for more than “100 kW generator,” because it tells the buyer you know what you have. The nameplate photo settles every argument.
No — standby sets have no overload allowance and are designed for ~200 hrs/year. Prime duty will destroy a standby set far ahead of its time.
Same machine, three duty cycles: standby (highest — occasional use), prime (middle — daily variable load), continuous (lowest — 24/7 constant load). The derate reflects mechanical reality.
ISO 8528 allows prime-rated sets 10% overload for 1 hour in every 12 — for transient peaks, not sustained operation.
Low-hour standby sets — the most common listing and the cleanest machines. Prime sets hold value with documented maintenance history.
Indirectly: emergency standby engines are exempt from Tier 4 Final; prime/continuous (non-emergency) engines are not. See the EPA tiers guide.
Nameplate (make, model, kW, serial), hour meter, and overall condition. The nameplate is the entire specification.
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