What is Prime Rated Power?

Helpfully, ISO-8528-1:2018 defines basic generator set rating categories based on four operational
categories: Emergency Standby Power (ESP), Prime Power (PRP), Limited-Time Running Prime (LTP) and Continuous Power (COP). In each category, a generator set’s rating is determined by maximum allowable power output in relation to running time and the load profile.

Incorrectly using these ratings can lead to lower generator life, invalid warranties and in some instances terminal failure.

How many hours can a Prime Rated Power generator run for?

So what is Prime Power? According to ISO-8528-1 a PRP-rated generator set must provide power for an unlimited number of hours per year, with reference to agreed operating conditions and crucially with maintenance intervals being carried out as per the manufacturers guidelines.

Typically an overload of 10% is allowed for 1 hour in 12, but this is not reflected in the ISO standard and therefore you should check with your manufacturer. This is typically used for regulation purposes and small unexpected loading.

What is the Prime Rated Power load Factor?

ISO-8528-1 states that the 24-hour average load factor is limited to 70 percent of the nameplate PRP rating. This means for every hour you spend at 100%, you should spend an hour at 40%, to give you an average figure. The load should also be variable (i.e. it goes up and down). If this is not the case, consider Continuous Power (COP).

if you use your generator for less than 250 hours per year, a standby rated (ESP) unit might be a better solution to lower your initial investment cost.

If you don't need to supply power at a for an unlimited number of hours, or have a fixed load profile? Consider some of the other ISO 8528-1 Ratings.

What alternative ratings are there to Prime Rated Power?

If the Prime Power Rating doesn't suit your requirements, you can consider buying a generator at an Emergency Standby Power (ESP), Limited-Time Running Prime (LTP) or Continuous Power (COP) rating.