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Department for Business, Energy and Industrial Strategy

Uninterruptible Power Supplies

1.1    Scope

Uninterruptible power supplies are used to allow electrical equipment to continue operating when the mains power supply is interrupted for a period, or the quality of the power supply deteriorates. They are widely used throughout industry and commerce to maintain the safety critical and business critical systems located in process control stations, computer rooms, data centres and server areas.

Uninterruptible power supplies are available with a wide range of different efficiencies. The Energy Technology List (ETL) Scheme aims to encourage the purchase of products with the highest efficiency.

The ETL Scheme covers two categories of products:

  1. Static (installed) uninterruptible power supply units or packages, with a power range greater than or equal to 10kVA, that use one or more electronic power converters, switches and energy storage devices (such as batteries) to generate their output voltage when operating without mains input power. 
  2. Rotary uninterruptible power supply units or packages, with a power range greater than or equal to 200 kVA that use one or more rotating electrical machines (i.e. AC generators) to generate their output voltage when operating without mains input power.

To be eligible for inclusion on the ETL, products shall meet the requirements as set out below.

1.2    Definitions

Uninterruptible power supplies are products that are specifically designed to maintain the continuity and quality of a power supply to electrical appliances or electrically driven equipment in the case of input power failure. When the mains electricity supply is operating, they charge up an energy storage device, which can be used to provide electrical power for a defined period when the mains electricity supply is interrupted 

1.3    Requirements 

1.3.1    Eligibility requirements  

To be eligible, products shall:

  • Be one of the following categories of product:
      a)    A static uninterruptible power supply as defined in BS EN 62040-3:2011 (or IEC 62040- 3: 2011).
      b)    A rotary uninterruptible power supply as defined in BS EN 88528-11:2004 (or IEC 88528- 11: 2004).
  • Include the following components (within the unit or package):
      a)    An electronic control system that controls the operation of the product.
      b)    Voltage inverter and rectifier devices (required for static uninterruptible power supplies, optional for rotary uninterruptible power supplies).
      c)    One or more energy storage devices (for example: batteries, flywheels, etc.) specified for use with the UPS.
      d)    One or more power supply filters.
      e)    A bypass switch (where required)
      f)    A motor generator set or alternator (for rotary uninterruptible power supplies only).
  • Be designed to be connected to, and to provide electrical power backup to, a three-phase electricity supply of nominally fixed frequency and voltage.
  • Have an appropriate Conformity Assessment mark.

Functionality criteria

To be eligible:

a)    Static UPS with more than one operating mode shall incorporate a high efficiency operating mode and include controls to switch between modes quickly and automatically when the utility supply falls below acceptable tolerances.
b)    Modular products shall incorporate controls to operate automatically at higher load per module, provided this demonstrably improves efficiency.

1.3.2    Performance requirements 

Eligible products shall:

a)    Meet or exceed the minimum efficiencies at full and part load conditions set out in Table 1.1 below, which depends on the product category.
b)    Have an input power factor that is greater than or equal to (i.e. >=) 0.93 at 25%, 50%, 75% and 100% of rated maximum power output at nominal input voltage.
c)    Have an input total harmonic distortion (THD) that is less than or equal to (i.e. <=) 5% at 100% of rated maximum power output. 

Table 1.1    Performance thresholds for uninterruptible power supplies


Product Category

Power range (kVA)

% of rated maximum power (i.e. % full load)





Static uninterruptible power supply units or packages











Rotary uninterruptible power supply units or packages











"<=" means "less than or equal to"
 ">" means "greater than"
">=" means "greater than or equal to"


1.4    Measurement and calculations

1.4.1    Measurement standards

Product performance shall be tested in accordance with the procedures and standard rating conditions laid down in the following standard:

  • Section and Annex J of BS EN 62040-3:2011 (or IEC 62040-3:2011): “Uninterruptible power systems (UPS) – Part 3: Method of specifying the performance and test requirements”.

1.4.2    Test requirements

With the following amendments:

  • Products shall be operated in their least efficient normal operating mode from a standard 230/400 Volt AC (+/-10%), 50Hz electrical power supply.
  • The package tested shall exclude additional isolation transformers that are not physically incorporated into the uninterruptible power supply unit or package, switchgear, low voltage switchboards, and generation sets.
  • Any static bypass switches fitted shall be in the ‘open’ position.

1.4.3    Rounding  

For the avoidance of doubt, test data should be presented to one decimal place. As an example, a rotary uninterruptible power supply product with an efficiency of 94.9 % when operating at 50% of its rated maximum power output would be deemed to be a fail.

1.5    Verification for ETL Listing 

Any of the following testing routes may be used to demonstrate the conformity of products against the requirements:

  • In-house testing – Self-certified
  • In-house testing – Self-tested and verified or cross-checked by an independent body
  • Witnessed testing
  • Independent testing
  • Representative testing (see clause 1.5.1)

Further information regarding the first four routes can be found in Guidance Note 5 on the ETL product testing framework1

1.5.1    Representative testing

Where applications are being made for two or more products that are constructed out of a number of identical power supply modules, test data may be submitted for a single ‘representative model’.  

1.6    Conformity testing

Products listed on the ETL may be subject to the scheme’s conformity testing programme in order to ensure listed models continue to meet the ETL requirements.