Pairing your UPS and standby generator: a complete guide

Power Control
06 Aug 2021

Uninterruptible Power Supplies (UPS systems) are used to protect sensitive IT equipment, emergency systems, medical equipment, evacuation systems and a vast range of other electrical loads from power supply fluctuations and short-term power loss. However, in order to protect electrical loads from longer power outages, it is common to provide a standby generator to work in conjunction with the UPS System(s).

What does pairing a UPS and Generator mean?

The UPS system provides an uninterruptible power supply so protects the load from power anomalies such as spikes, sags, surges, brownouts, noise and transitory failures, whilst also providing a battery backup time of anywhere from a couple of minutes to an hour or more.  When used with a standby generator the battery backup time is usually 5 to 15 minutes to allow enough time for the generator to start up and provide an alternative source of power to the grid.

During normal operation, an online double conversion UPS system continuously provides power to the load. Therefore, there is no gap in power or transfer time when the mains power source fails and the UPS batteries begin to support the load. The standby generator starts up when the automatic transfer switch (ATS) senses the loss of power and once stabilised, the load is automatically switched over to the generator.

As a UPS system can support a load for a short duration of time, the ATS delays its signal to the standby generator by approximately 10 seconds, preventing the generator from having to start up unnecessarily. In contrast, the ATS will keep the UPS and generator running for a period of time, usually minutes, after the mains power is restored to ensure stability.

Environmental Considerations

Planning for a UPS/generator installation can be a lengthy process. Once the compatibility of the systems has been established, consideration must then be pointed at the environmental acceptability of the Generator.

The process often starts with seeking local authority planning permission. As requirements vary according to location, local regulations must be checked before the installation can go ahead. Fuel storage is often a key factor and is subject to legal regulation in a majority of UK constabularies with a requirement for a fuel bund to catch any spilt fuel or oil and prevent fuel from escaping into the water. The base tank of a diesel generator is also double-bunded and contains fuel for between 8- and 24-hours runtime.

As with any diesel motors, backup generators create exhaust fumes which must be managed appropriately. Exhaust systems should minimise fumes and avoid disturbance to nearby people. If the exhaust pipes run through buildings, they should be lagged to minimise the problems of heat and fumes.

Audio pollution must also be kept to a minimum as generators may be running at any hours during the day and night. Acoustic housing with suitable noise attenuation ratings should be used. The ideal set-up for a generator is to be placed outside within a weatherproofed and noise reducing enclosure.

Sizing a UPS/Generator pairing

When pairing a UPS and generator, sizing is one of the most critical things to get right. In a facility’s electrical infrastructure there are typically two types of load; essential and non-essential.

Essential load: An essential load includes equipment that directly affects the ability of an organisation to operate and must be kept running without a break in power. These might include servers, computers, life safety and evacuation equipment and depend on the type of business, air-conditioning.

Non-essential loads: Non-essential loads aren’t fundamental to business operations and can be temporarily dropped in the event of a power failure such as general lighting and general heating, kitchen, hand dryers etc.

Classifying loads is important when pairing a UPS and generator as it must be decided which equipment will be supported by the UPS and generator and which equipment will go directly onto the generator.

Once categorised, the total load requirement of the generator can be determined. As a rule of thumb, generators should be sized to:

  • 2x the nominal UPS capacity for the typical UPS system using IGBT rectifier technology.
  • Add the additional non-essential load and allow additional capacity for ‘head room’

The UPS and generator specifier should be consulted for more accurate advice before the solution is purchased.

Ensuring Reliability of the UPS and Generator Pairing

Similarly to UPS systems, generators have intricate parts which must all work harmoniously to ensure neither fail when they are needed. In order to maximise the reliability of a diesel generator, they need to be well maintained, with an adequate fuel supply, a healthy battery and with sufficient coolant and oil.

Water jacket heaters can also prolong the life of a diesel generator as they are designed to keep the engine at an optimum temperature for start-up and load acceptance. The warmth is generated by heating and circulating the engine’s coolant.

Why Pair a UPS and Generator?

Generator/UPS pairing is used for numerous reasons. In some cases such as for critical systems including evacuation lifts, fire protection systems, standby lighting or medical and life support equipment, both a UPS and a generator must be installed to meet strict regulations and building codes.

For Data centres and facilities where sensitive information is being stored, a generator/UPS pair is used to keep the servers and IT equipment operational outside of the UPS autonomy.

Are some UPS more compatible with a Generator?

Walk-in feature

European UPS manufacturer, Borri SPA, has developed specialist features for their UPS systems that make them specifically compatible with generators. One of which is the walk-in feature whereby the load is transferred in steps from the UPS to the generator. The speed of this process is programmable resulting in different slew rates.

This prevents unnecessary strain from being put on the generator from start up. Longer start-up ramps (low slew rate) has a very low impact on the voltage and frequency of the diesel generator.

Another feature of the Borri UPS is the Diesel Mode. During Diesel Mode, the rectifier works at reduced DC voltage to prevent the UPS batteries from recharging. The input current is then reduced, resulting in a lower load on the diesel generator.

Borri SPA UPS systems also have a sequential start-up function. This programmable function allows for controlled load steps back to the AC mains and is generally used when several UPS systems are connected to the same network.

Diesel Mode

The next steps…

To be absolutely confident in the UPS/generator pairing, it is recommended to use a supplier with a wealth of knowledge and with many years of experience in installing these solutions. Power Control has been operating in the critical power space for nearly 30 years and has carried out a plethora of UPS/generator installations, commissioning and services. All of the technical sales team have years of experience and can provide advice and guidance on choosing the right UPS and generator systems to pair. Call 01246 431 431 or email for more information.