Matching a Generator to a UPS Load

Power You Can Trust


This paper seeks to highlight some of the main issues linked with closely matching a UPS load to a standby diesel generator. The issues outlined are at their highest where the load effects of the UPS are not diluted by other loads such as cooling etc.

Issues Highlighted

A UPS is a unique type of load and there are several factors which come into to play here when selecting a generating set to work alongside it.

Power Factor

A generator is of course a rotating machine, so the alternator is designed for max current to flow at 0.8pf lag i.e. 1000kVA, 800kW. The input power to a UPS when in normal operation usually runs at close to unity ie kVA = kW. It is immediately clear that you are not able to run generator and UPS at like for like kVA.

Power Input Considerations

There are two UPS power input considerations which are often overlooked, and these are operational losses of the system as a whole and battery recharge.

UPS losses

Like any other piece of equipment, the rating is the devices output rating not its input rating. Under normal running conditions an UPS will be at best 95-96% efficient. It will be less efficient at lower loads
hence these losses must be factored in. These losses also include battery trickle charge.

Battery Recharge

Following a mains failure it will be necessary for the UPS to recharge its battery system*…this is an additional power input requirement and it will vary:

  • From manufacturer to manufacturer
  • UPS rating to UPS rating
  • Size of UPS
  • Battery capacity to be recharged*
  • Depth of discharge etc. *recharge can often be delayed until mains has returned.

Two rules of thumb

  • Input current can increase by up to 20% following a prolonged period of discharge. This though does vary from UPS to UPS and often is a variable that can be managed
  • The minimum overrating of the generator to UPS (considering the variable elements) should be approximately 30-40% eg 100kVA UPS = 140kVA generator. This is a minimum.

Generator Load Factor

When matching a diesel generator to a load one consideration which is often overlooked is that of the generator load factor. The ISO standard for a Prime rated generating set stipulates that the “average load over a 24-hour, period should not exceed 70% of the generator capacity” ie a 1000kVA average load should not exceed 700kVA. It is also a requirement that the load should varying.

Operational UPS

If the UPS load is a varying one, the site isn’t prone to regular mains failures and the recharge requirement of the UPS do not exceed to generator max current then you can by and large minimise the consequences of the “load factor” in the selection calculation. Considering the worst case you have a 100/kW UPS + losses + normal trickle charge, running at full load for consistent periods supplied by a generating set running 24/7; a minimum generator rating of 170/180kVA would be recommended to support it. Typically larger power generating sets ≥ 1000kVA have a 75% load factor so this can be relaxed a little.

Other considerations

We now draw attention to three further primary considerations when planning an installation and these are:

  • Fault current / characteristics
  • Load harmonics
  • Load Power factor
  • Neutral Earthing arrangements

Fault Characteristics

If the UPS output sees a fast-rising leading edge of current, then it will typically switch to bypass (mains) in order to protect the inverter and offer a lower impedance mains source to help clear the fault. If the UPS is running generator supply at this time, then it is only the generator fault capacity that is available to clear the fault. We can provide fault clearance data on alternators and sets once you have selected your generator



All modern UPS are designed to operate with low levels of harmonic reinjection into the supply source i.e. less than 5% THDi and this is catered for should the UPS be running on generator. Ordinarily the UPS will largely deal with any harmonics presented to it by the load. If the UPS transfers to static bypass under fault conditions or on failure, the harmonic content of the load will need to be dealt with by the generator. The generator has limited ability to deal with large levels of harmonics – ideally it needs to be less than 5% THDi. In these circumstances oversizing the set helps.

Power Factor

When the UPS is in bypass and the generator is feeding the load, it will need to be capable of supporting the load power factor. Lagging power factor beyond 0.8pf lag will require derating of the generator output. Leading power factors should be avoided at all cost.


Neutral Earthing arrangements

Careful consideration needs to be given to the neutral earthing arrangements of both temporary and permanent generator installations. Providing continuity of both neutral and earth during mains to generator transfers is essential. As there are a number of possible scenarios here we recommend these are discussed with us.


General Considerations

Things to watch when comparing UPS manufacturers proposals:

  • That the kW output of the battery offered is matched to the kW rating of the UPS. Autonomy performance is directly linked to the amount of lead in the battery so a check of the physical battery weight will provide a good indication
  • When sizing the incoming supplies required, check for max input current and pf to the UPS this will vary from manufacturer to manufacturer
  • Input data should also show fault clearing capacity; This will vary from manufacturer to manufacturer