A generator advertised at 500 kVA might be a 500 kVA machine, a 450 kVA machine or a 400 kVA machine depending on which rating is quoted. The ratings describe duty, not marketing optimism, and comparing sets without establishing which one is in use is the easiest way to end up undersized.
The three ratings
Standby rating (sometimes emergency standby) assumes the set runs only during mains failure — typically a few hundred hours a year, at variable load, with no sustained full-load operation. It is the highest of the three figures.
Prime rating assumes the set is a primary power source, running long or unlimited hours at variable load, usually with a defined average load over any 24-hour period.
Continuous rating assumes constant full load indefinitely. It is the most demanding duty and therefore the lowest figure — often 20–30% below the standby rating of the same machine.
What happens if you choose wrongly
A set sized on its standby rating but run as a prime source will work, at first. Over time it consumes oil, builds carbon deposits and loses compression, and it reaches rebuild years earlier than it should.
This is the most expensive specification error in generator buying precisely because it does not announce itself. The set appears to work; it simply wears out early, and by then the decision is long forgotten.
Which rating applies to you
If the set exists solely for mains failure and will run a handful of hours a year, standby is appropriate. If it is a site's main supply, if the site has no mains connection, or if it runs scheduled hours for peak shaving, you need prime or continuous.
Rental and construction sets are almost always prime duty in practice, whatever the brochure rating says, because they are the only supply on site.
Tell us which duty applies when you enquire. It changes the recommendation more than almost any other factor.
Common questions
- How much lower is the prime rating than standby?
- Typically around 10% below standby, with continuous ratings often 20–30% below. The exact figures vary by manufacturer and model.
Examples from our generators range
A sample of what we list — the full range is larger, and we will confirm the specification with you before you order.
Portable Power Generators
Caterpillar XQ1750 Portable Power Module – 3516 DITA - 1750 kW
£150,000.00ex VAT
Request a quote for volume pricing
Portable Power Generators
Caterpillar XQ550G Natural Gas Power Module – G3512 TA - 555 kW
£150,000.00ex VAT
Request a quote for volume pricing
Portable Power Generators
Caterpillar XQ550G Natural Gas Power Module – G3512 TA - 555 kW
£135,000.00ex VAT
Request a quote for volume pricing