Generator Sizing and Compatibility for Uninterruptible Power Supplies

by Robin Koffler - Date: 2008-11-14 - Word Count: 615 Share This!

Most critical power protection solutions, incorporating uninterruptible power supplies (UPS), today are interfaced with an alternative source of back-up power (standby power) which could be a fuel cell or flywheel but more usually it is a diesel generator. Generator sizing and UPS compatibility are fundamental to power continuity and must be taken into account at the outset of any power protection plan.

Power Rating

A generator must be sized correctly so that when it's required to do so it will be able to power the UPS (taking into account any allowance for harmonics that the UPS's rectifier will generate) and the load/s that the UPS is supplying. Generators are typically rated in two ways:

Prime Power Rating (PPR) - whereby the generator supplies power as an alternative to the mains power supply, but on an unlimited basis.

Standby Power Rating (SPR) - whereby the generator supplies power as an alternative to the mains power supply but for a short duration, typically one hour out of every twelve.

A generator rated under SPR can be as much as 10 percent larger than one sized using PPR. This provides an overload capability for a short duration, perhaps to meet sudden load demand changes, for example.

For an uninterruptible power supply installation, PPR is the more suitable method of rating. It is extremely important, for achieving greater resilience (fault tolerance), that a generator and its UPS are suitably matched. Not only must a generator be able to accept the load of the uninterruptible power supply but the UPS rectifier and static bypass supplies must be able to operate with, and synchronise to, the output of the generator.

Generator set manufacturers have four recognised categories of load acceptance: one = 100%, two = 80%, three = 60% and four = 24%. Categories two, three and four are used in practice for PPR-rated generators. Load acceptance is closely related to the turbo charging system and the Break Mean Effective Pressure (BMEP) of the engine. This is a function of engine speed, number of cylinders and the swept volume of each cylinder.


For load acceptance to occur, a UPS must be able to synchronise to the voltage waveform supplied by the generator. Uninterruptible power supplies tend to have fairly wide input voltage windows and generator output is usually well within this. Its frequency, however, can vary, which can be problematic. This is overcome by widening the UPS operating parameters to accept a broader range. This may not always be sufficient, particularly for poorly maintained or undersized generators. Their output frequencies could drift and make it impossible for the UPS to synchronise.

A generator can never be matched on a 1:1 aspect ratio with an uninterruptible power supply. A UPS will at times be drawing additional current to charge its battery set. Generator sizing may also have to take into account the powering of essential loads, air-conditioning, for example, and emergency lighting. As already mentioned, a UPS rectifier can generate harmonics and this needs also be the taken into consideration when sizing the generator.

Ambient Temperature

The ambient temperature around a generator is important. It is usual for the engine room temperature to rise by around 10 degrees centigrade when a generator is in operation. Things can get quite hot if the outside temperature is also hot. High ambient temperatures can degrade generator performance and cause damage to turbo-chargers and exhaust systems. In such instances, it is normal to de-rate and increase the overall size of the generator installation.

Recommended practice is to oversize a generator by a factor of one-and-a-quarter to two-times the size of the uninterruptible power supply and to increase this to three-times or more when additional essential loads are to be powered.

Related Tags: generators, uninterruptible power supplies, power protection, riello ups, standby power

Robin Koffler is the General Manager for Riello UPS Ltd the UK subsidiary of Riello UPS (RPS S.p.A) a leading European manufacturer of Uninterruptible Power Supplies and a co-author of The Power Protection Guide(ISBN 978-0-9554428-0-3)- available from

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