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Department for Business, Energy and Industrial Strategy

Uninterruptible Power Supply

An uninterruptible power supply (UPS) is an electrical system that provides high quality electrical power without interruptions or power outages. Within the UPS system there are integrated storage systems such as batteries and flywheels which supply energy in the event of a power supply loss.

Key benefits of a UPS system:

  • Provides short-term power to a critical load (e.g. server room) during a power outage, allowing time for an alternative supply, such as a standby generator to be brought on-line.
  • Protects equipment by filtering a range of electrical disturbances, thus providing a clean power supply.

UPS units are commonly found in server rooms and data centres. They play a significant role in maximising the availability of systems.

New UPS technology, such as that listed on the ETL, can deliver an estimated 4% energy savings relative to the market average.

UPS units not only improve the quality of the electrical supply, but also smooth out any surges, spikes or dips in the power supply which could damage equipment.

ETL criteria updates

Updates to ETL technology criteria are as follows:

  • Efficiency thresholds of the ETL criteria have been increased over time to only include top performing products.
  • Static UPS must now incorporate a high efficiency operating mode and include advanced controls to switch quickly between modes
  • Modular products are required to incorporate advanced control systems to maximise efficiency.

Static & Rotary UPS

Two types of UPS are included in the ETL:

1.       Static UPS - used for supplying critical loads such as small data centres, and typically range in size from 10kVA to a maximum of 1MVA/unit for the industrial and commercial market. Static UPS are usually neatly housed in an electrical cabinet inside a building close to the electrical load and often use a battery storage system which is housed in the same cabinet.

2.       Rotary UPS - tend to be much bigger than Static UPS and individual units typically range from 200kVA up to 2.2MVA. Rotary systems are generally used to support high power requirements which, when operated as multiple units, can supply a critical load of 50MVA. Rotary UPS are often used in industrial or military applications.

Simple static UPS system

A static UPS usually consists of three main component parts:

1.       Rectifier/battery charger – this converts the mains supply AC voltage and current into suitable DC voltage and current needed to charge the battery and power the inverter.

2.       Storage unit – this is normally a battery which stores DC electrical energy and power for periods ranging from several minutes to many hours.

3.       Inverter – this converts the stored DC supply (from the rectifier or the storage device) into an AC voltage waveform – stabilised, filtered and regulated to supply the load.

Example: For two static UPS units or packages selected from the ETL of 100kVA, operating in parallel and supporting a critical 80kVA data centre load, with an efficiency 5% better than a non-specified product, the potential annual savings are calculated as:

  • £4,520
  • 40,540 kWh
  • 14 tonnes CO2e


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Rotary UPS

A rotary UPS uses flywheels and/or batteries as an energy storage device which provides short-term energy to the critical load in the event of a power supply loss. Rotary systems are used where the power system being supported is large and the potential for faults is high, due to their robust construction.

These devices also act as a buffer against power surges, spikes and dips. They are traditionally used in conjunction with stand-by diesel generators, where the storage device provides the back-up power for the brief time period required for the alternative supply to be brought on-line.

Example: Two rotary UPS units, both 500kW, providing critical energy supply support to an industrial process and critical IT infrastructure with a normal operational electrical load of 400kVA are selected from the ETL. The UPS units selected have an efficiency of 5% greater than a typical non-specified product. The potential annual savings are calculated as:

  • £28,220
  • 253 MWh
  • 89 tonnes CO2e

Hybrid Rotary UPS systems

Hybrid rotary UPS systems use a combination of static systems, flywheels and motor generator technologies to provide a robust and high power UPS system.

These hybrid systems can be very efficient and provide high levels of security.

When the supply from the utility network is good, the UPS operates by allowing the incoming electrical network to supply the critical load. In addition, a high efficiency motor/generator set is also online to provide supply back-up. If there is a short interruption or complete outage from the supply, the critical load is supported by a battery.

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