Prime (PRP) vs standby (ESP) power: which rating matters?

Standby power (ESP) is the maximum a generator can deliver during an outage, for a limited number of hours per year. Prime power (PRP) is what it can deliver for an unlimited number of hours under a varying load. If your generator runs several hours a day during load-shedding, size it on prime power.

What do the ISO 8528-1 definitions say?

The ISO 8528-1 standard defines the power ratings of generating sets. Our datasheets give two of them: prime power (PRP) and emergency standby power (ESP).

The standard also defines other ratings, such as continuous operating power (COP) for a constant load with no hour limit. They do not appear on our datasheets. The exact conditions vary by manufacturer, so always check the datasheet of the model.

What do our datasheets show?

On our water-cooled sets with Perkins engines, standby power is about 10 to 15% higher than prime power. The name of each set, such as "100 kVA", is its prime rating.

Values as printed on the datasheets. For the 13 kVA set the datasheet gives 11.4 kW standby, although 15 kVA × 0.8 would be 12 kW. See every model in our diesel generator range.

Load-shedding in Kinshasa: which rating should you use?

For a generator that runs several hours a day during load-shedding, use prime power (PRP). Daily use quickly exceeds the 200-hour annual limit of standby power: 2 hours a day already adds up to 730 hours a year.

Take the 100 kVA set with a Perkins 1104C-44TAG2 engine. It can deliver 110 kVA on standby. If it runs every day, count on 100 kVA.

Standby power suits a generator that rarely starts, during occasional grid outages. That is the case for a backup set in a building with an otherwise reliable supply.

Which rating does the finder use?

Our finder uses standby power only if you select "Standby (outages)" in the "Operating mode" setting. With "Prime / Main" or "Not sure", it compares your need with prime power, which is the more cautious choice.

When the mode is not "Prime / Main", the finder also suggests an automatic transfer switch (ATS) to transfer the load to the generator during outages. Use the finder to work out your generator size for your type of use.

Which rating do you use to estimate fuel use?

Prime power is also the basis for estimating fuel use. Our guide to fuel consumption in litres per hour gives the full-load figure for every set. For a house, see our worked example: What size generator for a house in Kinshasa?.

Know your appliances? The finder lets you work out your generator size and lists the suitable models.

Frequently asked questions

Can I run a generator at its standby rating every day?

No. Daily use is what prime power (PRP) is for. Under ISO 8528-1, standby power is intended for no more than 200 running hours a year.

Why is standby power higher?

Because it is only available for a limited time, with no overload allowed. The engine manufacturer allows this higher figure only for occasional use.

Sources: manufacturers' datasheets (Perkins engines, Leroy-Somer alternators), ISO 8528-1, MK Energies & Services finder calculation method.