Part 5 – Matching a generator to your UPS

In the 5th part of the IT Professional’s Guide to Standby Power, we look at the importance of matching the generator to your UPS, highlighting potential problems that IT decision makers can face, and identifying best practice.

Generators are not always included when specifying standby power for the protection of critical IT loads, however, once the decision has been taken to include a generator, it’s very important that the UPS and generator should be correctly matched.

Why do I need a generator?

Firstly, what role does a generator fulfil?

Standby generators are installed to supply power during lengthy power outages. This power is routed through the UPS to maintain extended runtimes to the IT critical systems. This removes the need for an orderly shutdown when batteries are low. It will also maintain the power supply to the essential systems during a protracted loss of mains power.

To cater for extended runtimes, most IT managers will choose a generator. This will maintain power for as long as there is fuel; a typical runtime before refuelling being 24 hours. For critical applications, a standby power generator is therefore as important as a UPS; usually, it is your only practical source of emergency power during an extended power outage.

If mains power fails, your UPS will accept the load and support the connected systems, usually sensitive electrical equipment or critical systems that can’t withstand a short break in power. The UPS acts as a power bridge, keeping critical systems operational until the generator synchronises with the UPS, and picks up the load.

Unacceptable frequency ranges

Consistent voltage and frequency are directly proportional to the size and type of generator. Standby generators generally specified for the protection of IT applications are smaller than generators that would support equipment within other areas of a building or manufacturing facilities. These smaller generators, unlike those used in utilities, produce less consistent power. Although the voltage range may be acceptable, the frequency range can often be wider than off-line or line-interactive UPS are designed to accept. As a result, it’s almost impossible to make an off-line or a line-interactive UPS work with a standby generator. Here’s why.

Off-line and line-interactive UPS technology

An off-line (including line-interactive) UPS passes mains power to the critical load. An off-line UPS may have some surge suppression and filtering but will have no power conditioning. A line-interactive UPS will have some power conditioning functions but will focus entirely on correcting voltage variations by means of a transformer and an automatic tap-switching feature. This has no effect on frequency variations, so the line-interactive UPS reacts to out-of-spec frequencies the same way as the off-line UPS, i.e. it will transfer to battery. When the battery is depleted, the off-line or line-interactive UPS will shut down the critical load.

Why on-line UPS technology works best with generators

On-line or double conversion UPS designs are ideal for use with generators because they accept input power with relatively wide variations in voltage and frequency. An on-line UPS actually re-develops the waveform. The AC input from either the mains or a standby generator is first converted to DC by the rectifier. The DC current is used to charge the battery. During an outage, DC current from the batteries is reformed by an inverter back to perfect sine wave AC for the critical load. Newer models also use power factor correction to provide an even wider input voltage range.

Because of this double conversion (AC to DC, then DC to AC), variations in the input frequency are of little concern. Therefore the same frequency variations that would cause an off-line or a line-interactive UPS to transfer to battery power have no effect on the on-line UPS.

Sizing the generator

A standby generator must be sized adequately for the UPS it is supplying.

As well as being capable of handling the step load of the UPS, the generator must have sufficient capacity to recharge the UPS batteries and cover the conversion losses of the UPS. The maximum step load of most generators is around 65% of the total rated capacity of the generator. Conversion losses and charging current can add a further 15% to the rated UPS power draw. A 100kVA UPS can, therefore, draw 115kVA and to accommodate a step load of 65% the generator must be sized at 172.5kVA, or 72.5% larger than the UPS.

It’s normal not to connect air conditioning equipment to the UPS, but instead to support it on the generator (since the air conditioning can handle a 30-second break while the generator starts). This load can be timed to reconnect once the step load of the UPS is supported, making full use of the generator capacity.

The UPS itself embodies a switched-mode power supply. This will create harmonics in any electrical circuit, but a supply of sufficient capacity will provide a ‘damping’ effect. Although this can be reduced by fitting a 12-pulse rectifier or using a unity input factor UPS, generous sizing of the generator will often provide a cost-effective alternative.

Running temperature

A further reason to allow plenty of generator capacity is that when running it is normal to experience a 10° rise in temperature within the proximity. When exposed to high ambient temperatures, generators will not run efficiently and can experience losses in power output.

High running temperatures can also cause damage to both the turbocharger and exhaust system. If you anticipate this situation, it is common practice to increase the overall size of the generator installation, since a generator will run cooler when not working to its full capacity. Good practice is to load the generator to 75% of its overall capacity for constant running. There is nothing wrong with running a standby generator at 100% of its output capability for short periods, but they are not generally designed for this. A ‘prime power’ generator should be specified where this is likely.

The generator circuit should also include two relatively inexpensive peripheral devices. The generator itself should be equipped with a good electronic governor. This will reduce the magnitude of frequency deviations on its output. There should also be an automatic transfer switch, ahead of the generator, which should have an in-phase monitor.

The load transfer switch prevents the generator power from back-feeding the mains supply. The in-phase monitor ensures that the two power sources (generator and mains power) are not out of synchronisation when mains is restored as the transfer switch cuts away from the generator in favour of mains. Out-of-sync power sources could result in a potentially destructive power spike to the UPS and unprotected connected equipment.

Larger Data Centres

As with UPS equipment, larger and/or more critical sites can utilise generators connected in parallel, either for capacity or redundancy. Use of (say) three generators where two can handle the load ensures that even during scheduled maintenance visits, full protection is available to the site. Synchronisation is achieved via the use of additional equipment, and generally, this can be retrofitted. This helps protect the original investment where growth has dictated an upgrade.


In summary, a matched UPS and generator for IT mission-critical applications requires an on-line UPS that can accommodate most frequency variations. There should also be a properly sized generator that will absorb most voltage variations. Further insurance can be provided by an electronic governor on the generator and an in-phase monitor on the automatic transfer switch.

The IT Professionals Guide to Standby Power  – contents

A Glossary of Standby Power Terminology

Part 1 – Planning your Standby Power Requirements

Part 2 – Potential Power Quality Problems

Part 3 – Sizing Generators

Part 4 – Specifying UPS

Part 5 – Matching a generator to your UPS

Part 6 – Making the right generator choices

Part 7 – How to ensure your batteries don’t fail

Should you wish to discuss any of the issues raised in these articles, please contact us.