Industrial generators for standby and prime power

A generating set is bought for a duty before it is bought for a size. A standby set runs only when the mains fails and is rated for limited hours at a varying load. A prime set is the main supply and runs as long as needed. The same engine and alternator carry a lower rating at prime than at standby, so two offers compare only at the same duty.

The load decides the rest. A data centre set picks up UPS systems and chillers in steps; a factory set starts motors. The largest single step, not the total, often sets the alternator and the governor response, so name it in the request.

Duty ratings

Definitions from ISO 8528-1 as manufacturers publish them; prime power also allows a 10 % overload for one hour in twelve. Makers define standby differently, one major range allowing 500 hours a year against the 200 above, so every offer is compared on the duty it actually states.

What gets specified

Generator ratings from 100 kVA to 3 MVA

kWe at a power factor of 0.8, the convention generating set ratings are stated at; current calculated as S / (√3 × V) and rounded to the nearest amp. A rating between two rows is quoted the same way. US buyers size sets in kW: a 200 kW set is the 250 kVA row, a 1 MW set the 1250 kVA row.

Permits in Great Britain: medium combustion plant and specified generators

In England and Wales the Environmental Permitting Regulations treat combustion plant of 1 MW to below 50 MW thermal input as medium combustion plant, whatever the fuel and however few hours it runs. Thermal input is taken on net calorific value. Plant first operated on or after 20 December 2018 needs its permit before it is commissioned. Existing plant above 5 MW has needed one since 1 January 2024, and existing plant of 1 to 5 MW needs one from 1 January 2029. At typical efficiencies, 1 MW thermal input is roughly a 500 kVA set.

Generating plant of the same size also falls under the specified generator controls. A backup set that runs only for emergencies on its own site and is tested for 50 hours a year or less is excluded from them. The Environment Agency is clear that it is still medium combustion plant and still needs its permit. Balancing services and demand side response do not count as emergency use, and testing beyond 50 hours needs the regulator's written agreement.

A new plant that runs 500 hours a year or less, as a three year rolling average, is exempt from the emission limits once a declaration is signed. Scotland moved its rules into the Environmental Authorisations (Scotland) Regulations 2018 on 1 November 2025, and Northern Ireland runs its own permit and specified generator regimes, so the site's nation belongs in the request.

Elsewhere in the EU, Directive (EU) 2015/2193 sets the same 1 to 50 MW band, and Regulation (EU) 2016/1628 covers sets mounted on mobile machinery rather than plant installed for its first use.

Transfer and control

2000kVA2887 A at 400 V2000 kVA generator1,600 kWe and 2,887 A at 400 V; one set or two in parallel, and a combustion plant permit.500kVA722 A at 400 V500 kVA generator400 kWe and 722 A at 400 V, close to the 1 MW combustion plant threshold.Automatic transfer switches and ATS panelsMains to generator transfer, 3 or 4 poles, open, delayed or closed transition.

What we need to quote

1. Duty rating and the rating in kVA, or the load to be carried
2. Voltage, frequency and the switchboard the set feeds
3. The largest load step: UPS, chiller or motor start
4. Fuel, and the runtime at a stated load
5. Enclosure, location and any sound limit
6. Stationary or mobile, and the expected running hours per year
7. Whether the set will ever earn from balancing services, which ends the backup exclusion
8. Transfer switch or synchronising, and the number of sets