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How to Select the Right UPS for Your IT Network Server

In the UK, the autumn and winter periods are characterised by higher demands on the electrical supply grid due to daylight saving and increased heating needs. Weather conditions can also put more stress onto the grid in terms of storms and nearby lightning strikes. Any of these issues can lead to a worsening in the quality of electricity supplied and momentary power outages. Together they create a ‘perfect storm’ that could knock out your IT facility.

Single Server UPS Installation

It is not uncommon in organisations and businesses with an IT closet or small computer room to protect their server with single UPS. The type of uninterruptible power supply installed will generally be a standby/off-line or line interactive system with a small built-in battery providing up to 5 minutes battery power.

The small battery power reserve should be enough to ride through momentary power outages. If a longer duration runtime is required, then the UPS must be oversized for the application or battery extension packs used where this option is available.

Use is also made of the communications port(s) built-into the UPS which may be USB or RS-232. The UPS may also feature a slot-in SNMP card, but this is more applicable to line interactive and online systems. Whichever communications option is used, signals can be provided to UPS monitoring software on the server to broadcast alarm message i.e. mains failure/on UPS battery and at a low-battery charge threshold, to initiate an automatic and unattended shutdown of the server.

Server Room Installations

In the server room, there may be one or more server racks (up to 42 or 47U in height) and the facility is more secure and managed than a small IT closet or server room. Where the UPS is best placed to protect the server(s) depends on their location and the electrical supply distribution within the room.

Uninterruptible power supplies are available in rack mount format up to 10kVA to allow them to be positioned inside a server rack. Tower models are generally floor standing and are designed for installation beside the server rack(s) or close to the distribution panel or board.

The larger the server room, the greater the need to consider a centralised or decentralised approach to power protection. In a centralised installation a single UPS system powers the distribution circuits to which all the IT loads and network devices are connected. This is referred to as the ‘critical power’ path in the facility and can run back to the building LV switchboard and incomer from the electrical sub-station. In a decentralised installation individual UPS are installed either inside each server rack or alongside the server racks.

A centralised UPS installation is easier to manage as it is a single installation. Future load optimisation and scalability can be an issue if a Monoblock design is chosen over a modular one. The electrical infrastructure must be sized for day-one and future load demands regardless of the UPS type.

Decentralised UPS may incur higher service and maintenance contract costs but may be more flexible in terms of meeting future load changes. Additional UPS can be installed into server racks or removed if the hardware within the network system is dialled back due to virtualisation or a move to Cloud.

One would expect server room UPS to be connected to the network via an IP connection and monitored over SNMP. Another typical feature of this size of UPS is the use of a maintenance bypass to allow the systems to be isolated for service and maintenance without disruption to the connected loads.

UPS for Datacentre Facilities

In a datacentre environment the UPS choice mirrors that of a server room in terms of a centralised or decentralised approach. A datacentre may also require additional redundancy paths for resilience and/or benefit from a local standby power generator.

The generator provides additional backup power when the mains power supply fails, allowing the uninterruptible power supply to conserve battery power. The generator also allows longer runtime periods for the fuel stored in its tanks (diesel or liquid petroleum gas – LPG).

For this size of installation, a case would need to be made as to whether to consider either a fixed capacity Monoblock or scalable modular UPS system and the levels of resilience required. Monoblock can achieve N+1 redundancy by adding installing a matching Monoblock UPS i.e. a 200kVA UPS with another 200kVA UPS. A modular UPS of 200kW comprising of four 50kW UPS modules can achieve N+1 redundancy by adding another 50kW module.

Energy efficiency also becomes more of an issue within a datacentre facility due to running costs. Most online Monoblock and modular UPS can achieve 95-96% online operating efficiency or greater. The UPS may also feature additional energy saving features including ‘sleep mode’ and ‘eco mode’. Sleep mode in a modular UPS system automatically only selects the number of modules required to support the load at any one time. Eco mode is a way to achieve 99% efficiency but with a reduced level of power protection and assumes a stable electrical supply.

For a datacentre UPS some installations look to purchase a system listed for energy efficiency on the Energy Technology List (ETL) in order to benefit from capital tax advantages for the organisation. More information:

Alternative Backup Power: Lithium-ion UPS Batteries

The traditional UPS battery is a sealed lead acid maintenance free type with a design life of 5 or 10 years. Some uninterruptible power supplies can be installed with a lithium-ion battery set. Lead acid batteries are temperature sensitive and require a 20-25˚C ambient to meet their working life expectations.
Lithium UPS batteries are less temperature sensitive, can last up to 10 years or longer and are more suited to rapid charge/discharge cycles. A lead acid battery recharges to 80% capacity within 24hours. A lithium battery can achieve this within 60-90 minutes on average.


Whatever the size of room or IT installation looking for uninterrupted power it is important to consider the quality of power required. The three UPS topologies standby, line interactive and online range in the power quality they can provide and power problems they can tackle without resorting to battery power. Online UPS which are considered to provide the ultimate level of power protection, also differ in their design. Modular UPS tend to be transformer-less and achieve their high operating efficiencies from the use of sophisticated power electronics. Monoblock UPS systems may be transformer-less or transformer-based with the transformer built-into the UPS and providing isolation (Galvanic) between the critical server loads and mains power supplies.

UPS maintenance post installation cannot be ignored either. An uninterruptible power supply requires annual maintenance if the system is to work when it is needed most. The alternative is a crashed system and a rush to find and plug-in emergency UPS supplies to get some of your servers and IT network back up and running.

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Computer and small server rooms tend to only have one or two server racks to protect from power outages. For these installations, a rackmount UPS if often the preferred solution. There are some factors to consider when planning to install an uninterruptible power supply in a server rack which do not apply to a floor standing tower system.

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