Power rating classification of standby generators — ESP, PRP, LTP and COP per ISO 8528-1

The guide on load-testing a standby generator covers how a generator set is tested under simulated load, but does not address the question of which power rating that set should actually be tested against. That question is answered by ISO 8528-1, which assigns internal-combustion generator sets not one, but four different power rating classes — each with its own combination of permitted overload, running-hours limit and average load factor, matched to a specific application.

The four power rating classes

Class	Full name	Running hours per year	Average load	Overload permitted
ESP	Emergency Standby Power	≤200 hours/year	~70% of ESP rating	No overload permitted
PRP	Prime Power	Unlimited	~70% of PRP rating (24-hour)	10% overload permitted for 1 hour per 12 running hours
LTP	Limited-Time Running Power	≤500 hours/year	100% of LTP rating	No overload permitted
COP	Continuous Power	Unlimited	Continuous 70-100% of COP rating	No overload permitted

Depending on which power rating class is applied, the same physical generator set can therefore carry several different power ratings on its nameplate — a set that achieves a given kVA rating as ESP typically delivers a lower power output when deployed as PRP or COP, because those classes require a structurally higher load over a longer service life.

ESP — standby power for grid interruptions

ESP is intended for applications where the generator set only steps in during a grid outage, with a strictly limited number of running hours per year (typically ≤200 hours) and no overload permitted. This is the class that applies to most standby power supplies in building installations falling under NEN 1010 — the set runs almost exclusively during actual grid outages and periodic load tests.

PRP — prime power without the utility grid as backup

PRP is intended for applications where the generator set is the sole power source, without the utility grid as backup, with a variable load and an unlimited number of running hours per year. An average load factor of roughly 70% over 24 running hours is assumed, with a permitted overload of 10% for up to 1 hour per 12 running hours to absorb load peaks.

LTP — limited time at full power

LTP is intended for a generator set that can run continuously at 100% of its stated power for a limited number of running hours per year (typically ≤500 hours), with no possibility of further overload — often applied for temporary or peak-power applications requiring a higher constant power than ESP, but not the unlimited service-life loading of PRP or COP.

COP — continuous power as a permanent power source

COP is intended for applications where the generator set runs continuously, at a constant load between 70% and 100% of its stated power, for an unlimited period — the heaviest class in terms of service-life loading, applied for permanent power generation without a grid connection.

Why the choice of class matters

A generator set sized according to the ESP class but in practice deployed as PRP — for example during a prolonged grid outage where the set forms the sole power source for weeks — runs structurally more heavily loaded than it was specified for, with an increased risk of premature wear of the engine and generator. Conversely, oversizing a generator set based on the COP class for an application that is actually ESP leads to unnecessary investment and fuel costs.

Practical relevance

When selecting or assessing a standby generator, it is relevant to check which ISO 8528-1 power rating class is stated on the nameplate and in the supplier's specification, and whether that class actually matches the intended usage pattern — including the expected number of running hours per year and whether the set is the sole power source or a backup to the utility grid.

Common mistakes

Applying the ESP rating to an application that is actually PRP or COP — a prolonged grid outage in which the set acts as the sole power source can far exceed the ESP load limit.
Comparing power ratings from different classes with each other without correcting for the class difference — the ESP rating of a set is typically higher than the PRP or COP rating of the same physical set.
Confusing the permitted PRP overload with a permanent power reserve — the 10% overload applies explicitly for up to 1 hour per 12 running hours, not for continuous use.
Not accounting for the running-hours limit of ESP (≤200 hours/year) or LTP (≤500 hours/year) in an application where the set operates more frequently than expected, for example due to regular, prolonged grid interruptions.