The Ultimate Beginners’ Guide to Uninterruptible Power Supplies [UPS]

Datacentres contain a host of sensitive IT equipment which are hungry for electrical power. Their importance to the life of an organisation cannot be overstated; serious problems happen when datacentres go offline. So how can you ensure your datacentre always works 24 hours a day, 7 days a week, 365 days a year – no matter what? The answer lies in Uninterruptible Power Supply (UPS) systems.

What is a UPS?
Why are UPS Systems Important?
How much do UPS systems cost?
What kinds of UPS are there?
How do I go about choosing a UPS that’s right for my datacentre?
What happens if the UPS fails?
How can I prevent UPS failure?
Glossary of terms

What is a UPS?

A UPS system is a device positioned within the datacentre ready to supply power to critical IT equipment in the event that the main electrical power supply is cut.  As such, a UPS is an essential part of any business continuity strategy.

Why are UPS systems important?

The primary reason for deploying a UPS system is to provide a kind of insurance policy against complete power failure or intermittent power failures from the grid. In this way, UPS systems ensure that the IT processes and workflows being undertaken by datacentre equipment are not disrupted, and operations can continue as normal.

UPS systems cannot last forever; they are designed to continue providing power for a defined period of time.

Operating a datacentre without a UPS leaves you open to sudden, unexpected downtime that may last for indeterminate period. Withdrawing power from sensitive IT systems in an uncontrolled manager without warning can result in damage to those systems and make it difficult to restart them effectively. Not only that but, without a UPS, you have no backup power supply available to you to reboot IT systems and get operations back up and running.  You literally have to wait for main power supplies to come back on stream

How much do UPS systems cost?

UPS systems vary in cost according to their rateable size i.e. how much power they are capable for producing and for how long (runtime).

The larger the requirement, the more expensive the UPS system will be to purchase.

 A small, sub 1kVA UPS capable of running a server and several other appliances in a small office could easily cost less than £1,000. Large systems running into the hundreds of kVA could equate to tens of thousands of pounds, or more once other, related costs are added.

Other factors impacting on cost would be the kind of UPS technology used and any unusual design characteristics such as dense form-factor or ability to operate in high temperature environments.  Many UPS deployments also feature multiple UPS units for redundancy, effectively multiplying the procurement cost.

Of course the purchase price of a UPS system is only part of its whole-life cost. Other considerations include:

  • Shipping (UPS systems are typically very heavy)
  • Overall solution design
  • Installation and commissioning
  • Servicing and maintenance
  • Consumables e.g. batteries, fuel

What kinds of UPS are there?

There are three main UPS technologies:

Battery UPS

Most UPS systems operate on the principle of using standby batteries to provide a set runtime period against a given load. The batteries are permanently online and available to autonomously take over the power supply for the datacentre once a power drop or failure is detected. All battery-based UPS systems require their batteries to be periodically replaced in line with a maintenance cycle; large systems allow for hot-swappable battery components so replacements can be undertaken without the UPS being withdrawn from service.

Rotary/Flywheel UPS

Flywheel-based rotary UPS systems use stored energy in the form of a high-mass spinning flywheel using magnetic bearings. In event of power drops or failures, these can deliver valuable seconds of immediate power to the datacentre infrastructure to avoid disaster and enable sufficient time for an alternative power source such as a diesel generator to be brought online.

Fuel Cell UPS

Fuel cells are essentially batteries that do not run out of energy so long as they are fed with sufficient fuel. Fuel cell UPS systems are rare and recently developed, but are likely to become increasingly popular in future. Energy is created from chemical reactions resulting from changes to the fuel produced within a specially-designed polymer exchange membrane. Fuels include hydrogen, phosphoric acid, solid oxide and molten carbonate.

How do I go about choosing a UPS that’s right for my datacentre?

There is a wide selection of UPS systems on the market to cater for every need. However, knowing which one to implement needn’t be a difficult process. Here are three key areas to consider:


Undersizing your UPS can have disastrous consequences. Oversizing can be a waste of money and result in an inefficient system.

Sizing your UPS correctly ensures that you have enough power to supply to mission-critical datacentre infrastructure should you suffer an outage, and that you are maximising your budget.

Sizing your UPS properly means finding out what your power requirements are. Here is a simple checklist to follow:

  1. Write down each piece of equipment your UPS needs to protect
  2. Find out how many amps and volts each uses
  3. Multiply the two numbers together to calculate VA
  4. Add up all the VA figures for the devices on your list

Most UPS systems are rated in kVA (thousands of VA) so be sure to look for one that accommodates your peak requirements plus some extra headroom.

Single phase vs. 3-phase power

Your datacentre will either be served by single phase or 3-phase power. This is important because UPS platforms are purpose-built to operate in one or the other. Ask your facilities/estates manager, your utility provider or a qualified electrical contractor if you aren’t sure which your environment is being powered by.

If your datacentre currently consumes in excess of 20kW (similar, but not the same as 20kVA) then you will find it difficult to find a manufacturer that offers a standalone single-phase power UPS platform rated this high.

If you need to replace an existing UPS then a straight swap (i.e. single phase for single phase; 3-phase for 3-phase) will make the most sense.

However, if you are currently on single-phase power and your power requirement is growing, you may need to change your underlying power distribution as well as your UPS in order to achieve the resilience and reliability you are looking for.


Energy needs associated with IT usage can be unpredictable and it is a constant challenge to plan for change without compromising power protection.

The answer lies in operating a UPS infrastructure with sufficient scalability to cope cost-effectively and non-disruptively with growing power demands.

A common mistake is specifying a UPS platform without considering future growth requirements as this can lead to that UPS needing to be replaced with a larger system before its planned end-of-life.  The best way to avoid overcompensating with a system that is too large, is to opt for a modular approach to UPS scale-out.

UPS platforms that allow you to grow capacity in increments of, say, 10kVA not only make growth more manageable and cost-efficient; it also ensures that UPS platforms can be run at the upper end of their utilisation level which is key to maximising energy efficiency.

Modular systems are also central to building redundancy into UPS solutions, so that risk of failure is spread across multiple systems rather than a single, high-risk dependency for your entire IT infrastructure.

What happens if the UPS fails?

The nightmare scenario for any IT leader is that a safety mechanism that is intended to protect IT infrastructure in the event of failure, fails.  Though unlikely, this can happen with UPS systems, and result in catastrophic business consequences such as lost revenue and a tarnished reputation because of prolonged, unexpected downtime.

For many organisations, this risk, albeit small, is too great to be ignored. In such circumstances, businesses look to implement a layer or redundancy into their UPS infrastructure to mitigate against component or system failure. These approaches are likely to take one of two forms:

N+1 Redundancy

N+1 denotes a marginal level of redundancy and can be applied to any IT infrastructure, not just UPS.  It protects against the failure of a single component by always having a spare component available ready to take over in an automated fashion.

For example, in a UPS setup requiring four 20kVA UPS systems to provide the necessary power protection, an N+1 architecture would operate five. Should any of the UPS systems unexpectedly fail, the remaining systems are sufficient to keep the required level of protection.

This level of redundancy is also helpful in maintenance situations, allowing servicing to be carried out on a rotational basis without ever compromising the integrity of the UPS power protection available.

2N Redundancy

2N is a more ‘belt-and-braces’ approach to redundancy, and involves entirely duplicating the required primary UPS protection in a secondary, parallel infrastructure.  This ‘mirrored infrastructure’ approach protects against the failure of an entire UPS system and is sought out by the most risk-averse organisations.  It is also more likely to be used in UPS infrastructures with little or no modularity i.e. a single 50kVA UPS sitting ready to take over in the event that the ‘live’ 50kVA UPS fails.

How can I prevent UPS failure?

UPS systems are based on mature, stable and proven technologies. Those produced by the leading manufacturers are typically very reliable and covered by warranty for at least one year.

That being said, UPS failures can occur and the probability of this is heightened by several factors:

  1. Inappropriate UPS sizing
  2. Poor UPS install/design
  3. Age of UPS system
  4. Sub-optimal UPS environmental conditions
  5. Insufficient UPS maintenance

Inappropriate sizing

The efficiency and reliability of your UPS system can be affected by its load utilisation and the use of appropriately sized components.  UPS systems cannot be expected to take on power loads that exceed their ratings, or push utilisation to 100% (80-90% is ideal).  Similarly, battery-based UPS systems that do not use approved battery components may be more prone to failure or lack of performance.

TIP: Make sure you keep your UPS right-sized as your power demands change over time

UPS age

Arguably the most important components of a UPS system are consumables.  For the most part this means batteries, and batteries degrade over time – compromising the reliability of the UPS system if they are not regularly checked and replaced.

In other respects, UPS systems can continue to operate effectively despite their advancing years, but only if they are regularly maintained to professional standards.

The evolution of new UPS technology continues to drive higher standards in efficiency, reliability and performance, and this is probably the biggest reason why most organisations choose to replace their entire UPS system to the latest generation of technology every 4–5 years.

TIP: Don’t just track the age of your UPS assets, but the consumables within them too.

Environmental conditions

Temperature and humidity can be serious issues for UPS reliability, unless the UPS system chosen has been specially designed to cope with more extreme, industrial conditions.

For the most part, UPS systems are intended to operate under environmentally-controlled conditions that do not range higher than around 24oC (75oF), hence why datacentre cooling solutions are important for system efficiency.  Even the smallest tower-based UPS systems require a minimum surround distance in order to benefit from passing airflow.

The other enemy of UPS reliability is moisture, both in the form of water entering the datacentre environment at ground level, and air-moisture which condenses upon hot equipment and seriously compromises the integrity of electrical circuitry and causes corrosion.

TIP: Consider using an automated DCIM solution to ensure that datacentre temperature and humidity levels never reach levels that could compromise UPS reliability

UPS Maintenance

Financially-ruinous downtime could be the least of your worries with a poorly maintained UPS system. Failing to carry out the appropriate UPS maintenance processes could result in serious injury or even death.

A robust UPS maintenance programme can be undertaken internally or outsourced to a qualified third party.  Combining regular inspections with periodic testing and replacement cycles, the ultimate maintenance regime will address the following areas:

  • A proactive, rather than reactive posture where maintenance is used as a positive springboard for continual improvement opportunities
  • Properly assigned and documented responsibilities for weekly, monthly, quarterly and annual maintenance tasks
  • Maintenance standards set to your business objectives and expectations – not merely the minimum recommended by the manufacturer
  • Clearly understood boundaries for when and where external UPS maintenance/servicing skills may be needed to complement your own
  • A commitment to always using UPS manufacturer-accredited parts and skills, rather than off-brand components of uncertain provenance and generic electrician labour
  • Investment in remote monitoring and/or DCIM (Datacentre Infrastructure Management) technology to give you a holistic and automated, 24/7 view of UPS health.

Glossary of Terms

2N Redundancy

A level of redundancy equating to a fully mirrored system. In a 2N configuration, the entire infrastructure requirement of multiple components can be fulfilled by an entire secondary infrastructure waiting to take over.

3-Phase Power

The most common method for the transmission and distribution of alternating current (AC) electrical power, the three phases or ‘waves’ of current produce more reliable and constant power output more efficiently at higher quantities than single-phase power.


Datacentre infrastructure management (DCIM) solutions are used to monitor and control both IT and facilities management metrics within a single console.


1,000 volt-amps. Volt-amps (VA) are the unit of measurement for actual or apparent power.  The VA of an appliance is calculated by multiplying its voltage to its operating current.


1,000 watts. Watts are the unit of measurement for real electrical power.

N+1 Redundancy

A marginal level of redundancy enabling an infrastructure to continue operating despite the failure of an essential component. In an infrastructure with four components, N+1 would operate five.

Power Distribution Unit (PDU)

A PDU is used within datacentre environments to distribute power safely and efficiently to the necessary racks and cabinets containing IT equipment of different power ratings.


Power Usage Effectiveness is a ratio expressing the efficiency of total power delivered to a datacentre facility to be used by the computing equipment within in.  The lower the PUE (1:1 would be the lowest theoretically possible), the more efficient the datacentre is at converting its electricity consumption into value-generating IT-driven activity.  Cooling datacentre IT equipment is typically the greatest challenge to achieving a low PUE.

Single-Phase Power

Single-phase power is supplied exclusively to domestic electricity customers rather than ‘industrial’-sized consumers with greater demand who receive 3-phase power.  See 3-Phase Power.


Uninterruptible Power Supply. A battery-based hardware platform that provides a reliable and appropriate level of electrical power – typically to IT systems / datacentres – in the event that mains power is lost.


The track record of availability performed by IT systems over a given period. Uptime is expressed in percentage terms (e.g. 99.999% uptime) and normally covers one year.

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