UPS: Uninterruptible Power Supply
A guide to energy efficient equipment listed on
the Energy Technology List (ETL)
Contents
1. Introduction to the Energy Technology List (ETL)
2. Setting the scene – Uninterruptible Power Supply (UPS)
3. UPS - technology improvements
4. UPS equipment eligible for the ETL
• Static UPS
• Rotary UPS
• Hybrid Rotary UPS
5. UPS operating modes
6. Parallel operation of UPS
7. Further information
1
Introduction
The ETL is a government register of energy saving products. When you select products from the list you are
choosing from amongst the most energy efficient products in the marketplace.
When replacing equipment, businesses are often tempted to opt for equipment with the lowest capital cost.
However, such immediate cost savings may prove to be a false economy. Considering higher energy efficient
products, means that life cycle costs are reduced, improving cash flow in the longer term.
Businesses can also claim accelerated tax relief through the Annual Investment Allowance (AIA) for
investments in plant and machinery equipment. The AIA has been temporarily increased to £1 million from
January 2019.
This leaflet illustrates the benefits of investing in UPS energy saving equipment which qualifies for the ETL.
The ETL comprises two lists:
• Energy Technology Criteria List: defines the performance criteria that equipment must meet to qualify
for the ETL;
• Energy Technology Product List: is the list of products that have been assessed as being compliant with
ETL criteria.
Eligible UPS products on the ETL can be searched at:
https://etl.beis.gov.uk/engetl/fox/live/ETL_PUBLIC_PRODUCT_SEARCH
2
Setting the scene – Uninterruptible Power Supply (UPS)
Definition
An uninterruptible power supply (UPS) is an electrical system that provides high quality electrical
UPS units not
only improve power without interruptions. The mains electrical power supply is connected to the input of the UPS and the
the quality of output is connected to the customer electrical load. Within the UPS system there are integrated storage systems
the electrical such as batteries and flywheels which supply energy in the event of a power supply loss.
supply, but
also smooth Key benefits of a UPS system:
out any • Provides short-term power to a critical load (e.g. server room) during a power outage, allowing time for an
surges, spikes alternative supply, such as a standby generator to be brought on-line.
or dips in the • Protects equipment by filtering a range of electrical disturbances, thus providing a clean power supply.
power supply
which could UPS units are commonly found in server rooms and data centres. They play
damage a significant role in maximising the availability of systems. Did you know?
equipment. New UPS technology, such as
UPS modules are often operated in parallel to increase availability and
that listed on the ETL, can
provide extra security of electrical supply to the connected equipment. One
deliver an estimated 4%
or more additional modules are included to maintain capacity in the event
energy savings relative to the
of a failure. This is known as operating in ‘redundant configuration’. Under
market average.
these circumstances, each UPS shares the supply but operates at a reduced
power level. Or some modules operate at high capacity and others are
inactive until needed.
3
UPS - technology improvements
Since 2014 there have been significant increases in the efficiency of UPS units. The ETL technology
criteria have been updated to reflect this:
• 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.
• Whole system (UPS & storage device) needs to be compliant to be eligible for
the ETL.
© Vertiv
4
UPS equipment eligible for the ETL:
q
Static & Rotary UPS
Two types of UPS are included in the ETL.
Static UPS An ETL listed UPS must meet defined energy
Static UPS are used for supplying critical loads such as small data efficiency levels under various load conditions. In
centres, and typically range in size from 10kVA to a maximum of this document, the baseline scenario below, has
1MVA/unit for the industrial and commercial market. been used to calculate the potential financial (£),
energy (kWh) and carbon savings (tonnes CO2)
Static UPS are usually neatly housed in an electrical cabinet inside unless otherwise indicated:
a building close to the electrical load. They occupy much less • UPS operates continuously, 8,760 hours a year
space than rotary systems and often use a battery storage system • All UPS operate at unity power factor
which is housed in the same cabinet. • Price for electricity* 11.14p/kWh
• Carbon emissions* for electricity 0.35156
kgCO2/kWh
Rotary UPS
* BEIS 2017
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.
© Riello 5
Important
Static UPS Static UPS must now incorporate
a high efficiency operating mode
Static UPS systems are eligible for the ETL and include advanced controls to
switch quickly between modes to
be ETL listed.
Simple static UPS system
Lithium-ion
batteries offer
a number of
benefits over a
valve regulated
lead acid
battery (VRLA)
including faster
charging,
slower battery
discharge and
less battery
replacements.
© Eaton – Power Quality Products 6
Static UPS
Static UPS systems are eligible for the ETL
Simple static UPS system
Simple static UPS system Example: For two static UPS units or packages
A static UPS usually consists of three main component parts: selected from the ETL of 100kVA, operating in
parallel and supporting a critical 80kVA data
1. Rectifier/battery charger – This converts the mains supply AC voltage centre load, with an efficiency 5% better than
and current into suitable DC voltage and current needed to charge the a non-specified product, the potential annual
battery and power the inverter. savings are calculated as:
2. Storage unit – This is normally a battery which stores DC electrical • £4,520
energy and power for periods ranging from several minutes to many hours. • 40,540 kWh
The most common battery used by UPS manufacturers is the sealed or • 14 tonnes CO2e
valve regulated lead acid battery (VRLA). Increasingly, Lithium-ion batteries
are now being used. Although they have a higher capex cost, these With a typical additional capital cost of
batteries are said to offer a much lower cost over the battery lifetime. £16,000 (£56,000 non-ETL capital cost) and
Energy storage for UPS can also be provided by very high speed flywheels, lifetime energy and AIA benefits of over
or a combination of batteries and flywheels. When required, the storage £50,000 at today’s prices, the financial
unit provides the electrical supply to the critical load through the inverter. benefit of choosing an ETL listed product is
over 3 times the additional cost. Furthermore
3. Inverter – This converts the stored DC supply (from the rectifier or the with a potential AIA of £13,680 in year 1 plus
additional energy savings, the extra capital
storage device) into an AC voltage waveform – stabilised, filtered and
cost is recovered in under 2 years.
© BPC regulated to supply the load.
7
Rotary UPS
Rotary UPS systems are eligible for the ETL
Rotary UPS system Example: Two rotary UPS units, both 500kW,
A rotary UPS uses flywheels and/or batteries as an energy providing critical energy supply support to an
Rotary systems,
storage device which provides short-term energy to the industrial process and critical IT infrastructure with
when operated a normal operational electrical load of 400kVA are
as multiple units critical load in the event of a power supply loss.
selected from the ETL. The UPS units selected have
can supply a an efficiency of 5% greater than a typical non-
critical load of These devices also act as a buffer against power surges,
specified product. The potential annual savings are
50MVA. spikes and dips. They are traditionally used in conjunction
calculated as:
with stand-by diesel generators, where the storage device
Rotary UPS are provides the back-up power for the brief time period • £28,220
often used in required for the alternative supply to be brought on-line. • 253 MWh
industrial or • 89 tonnes CO2e
military The rotary UPS is generally reserved for applications that
applications. require more than 200kVA of protection, for example high With a typical additional capital cost of £40,000
power requirements in industrial or military applications. (£200,000 non ETL capital cost) and lifetime energy
Rotary UPS systems are normally extremely large and and AIA benefits of nearly £300,000 at today’s
heavy systems which require routine maintenance. prices, the financial benefit of choosing an ETL
listed product is over 7 times the additional cost.
Rotary systems are used where the power system being Furthermore with a potential AIA of £45,600 in year
supported is large and the potential for faults is high, due 1 plus additional energy savings, the extra capital
to their robust construction. cost is recovered within a year of purchase.
8
Rotary UPS
Rotary UPS systems are commonly combined with a back up diesel generator
Rotary UPS featuring flywheel,
generator and diesel engine
B Flywheel
A
C
Rotary UPS system
In the figure above, when mains supply is available breakers DA, D1, D2 and DB are closed feeding the critical load. The
automatic bypass D3 is open.
The motor/generator (A) acts as a motor and drives a flywheel providing rotating energy storage. The motor/generator acts as
an ideal power factor correction device and filters out minor transients in the supply.
If there are any micro breaks in supply, the flywheel can maintain the supply to the critical load. If there is a longer break in
electrical supply, the input breaker D1 opens, the diesel engine starts (B), the clutch closes (C) and the critical supply is
maintained through the generator all within seconds. There is a manual isolator (I3) that allows the unit to be bypassed.
9
Hybrid Rotary UPS systems
Hybrid Rotary UPS systems are eligible for the ETL
Hybrid Rotary UPS system
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.
The figure on the right shows that 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.
Systems of this type are eligible for the ETL.
10
UPS operating modes
There are three main modes of operation. ETL listed products must meet the ETL criteria when
tested in their least efficient mode.
The VFI mode is 1. VFD (voltage and frequency dependent) – commonly known as ‘off-line’, is a UPS where the
the most common
as it is the only mains power supplies the critical load directly during normal operation.
true provider of
power without
interruption.
2. VI (voltage independent) – commonly known as ‘line interactive’, is a UPS similar to the off-
line system except that it offers a higher performance by adding voltage regulation features in
The VFI mode is the by-pass system, often by installing a ‘buck-boost’ transformer.
the most
appropriate for 3. VFI (voltage and frequency independent) - more commonly known as ‘on-line’ or ‘double
computer and data
centre applications conversion’. In this system, the output is always isolated from the input mains supply, and
as it is therefore any mains fluctuations or disturbances are only seen by the rectifier. The
independent of components of an on-line UPS are always active, and therefore need to be much more robust
both variations in
mains supply than those used for an off-line UPS which is only asked to provide output intermittently. Many
voltage and systems now go beyond a single on-line system and provide parallel or redundant
frequency. configurations. Often a UPS system is made up of a number of UPS units, connected in parallel.
11
Parallel operation of UPS
Parallel operation is used to enhance system reliability
In large businesses where reliability is of great importance, a single UPS can also be a single point of failure that can
cause major disruption. To provide greater reliability, multiple smaller UPS units and batteries can be integrated
together to provide redundant power protection equivalent to one very large UPS. Operating in this way is called an
In operating an ‘N+1’ system.
‘N+1’ system, Typical efficiency/load curves for transformer and transformer-less UPS systems*
where if the load
can be supplied by The figure shows the efficiency of a
N units, the UPS compared to the electrical load.
installation will As shown, the efficiency drops off at
contain N+1 units;
failure of one unit lower loads, leading to increased
will not impact on energy loss.
system operation.
Higher efficiency UPS, such as those
In normal specified on the ETL, are designed to
operation, each minimise the energy losses at all
individual UPS unit load levels.
operates at 50%
load or less.
For example, if two 200kVA UPS units were connected in parallel supplying a 200kVA load. Under normal operation
they share the load equally at 100kVA each; However, if there is a problem with one UPS unit, it can be taken out of
service for repair or maintenance while the other UPS is capable of supplying the load on its own.
* Source data: The Handbook Uninterruptible Power Supplies, Peter Bentley. 12
Where can I find more information?
For information about the ETL please visit: https://www.gov.uk/guidance/energy-technology-list and
see our Information for Purchasers factsheet:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/6
86850/Key_Information_for_Purchasers_Factsheet__February_2018.pdf
Or contact the ETL Help Line on 0300 330 0657; or email ETLQuestions@carbontrust.com
To search for a product on the ETL please visit:
https://etl.beis.gov.uk/engetl/fox/live/ETL_PUBLIC_PRODUCT_SEARCH
© Crown copyright 2020
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Government Licence.
To view this licence, visit www.nationalarchives.gov.uk/doc/open-government-licence/ or write to the Information Policy
Team, The National Archives, Kew, London TW9 4DU, or email: psi@nationalarchives.gsi.gov.uk. 13
