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How Long Will A UPS Provide Backup Power For?

The primary role of an uninterruptible power supply is to provide backup power when there is a power outage, or the mains power supply becomes unstable, to keep the connected loads running. The amount of backup power available is dependent upon several factors.

UPS Lead Acid and Lithium-ion Batteries

When the mains power supply is present, an uninterruptible power supply will charge its energy storage component. The most common way to store energy in a UPS is to use a battery and the most installed is a valve-regulated lead acid (VRLA) battery. This type of battery technology is used for standby power applications including UPS systems, emergency lighting, security, fire, and alarm panels.

Lithium-ion battery usage with uninterruptible power supplies is a new application. The principle role is still that of ‘energy storage’ but using lithium-ion (Li-ion) in place of lead acid battery technology provides some key advantages. Lithium-ion batteries are more suited to applications requiring fast charge/discharge cycling and can recharge in 2-3 hours compared to the twenty-four or more sometimes required for lead acid. Lithium-ion batteries are more compact and heat tolerant.

Lithium-ion batteries have a higher cost price than lead acid but for some UPS applications, this can be justified. They also have a longer working life than lead acid. A lead acid battery will have a 5year or 10year design life and will require replacement in years 3-4 or 7-8 depending on usage and environmental factors such as ambient temperature. A lithium-ion battery can have a design life of 10years or greater.

There are other sources of energy storage for uninterruptible power supplies including supercapacitors and DC flywheels, but these tend to offer far shorter duration runtimes, measured in milliseconds rather than minutes or hours and require additional support from a local standby power generator (diesel or liquid petroleum gas).

Standby generators can also be used to provide additional backup to UPS systems installed with either VRLA (lead acid) or lithium batteries. This type of arrangement is common where a runtime of several hours is required, or the load size is large, and a generator provides a cost-effective alternative to a large battery set.

Most UPS manufacturers and suppliers now offer both lead acid and lithium-ion powered UPS products in discrete product sizes and runtimes.

How To Size a Single-Phase UPS in VA or Watts?

When you look at a range of UPS, they are either sized in VA (Volts x Amperes) or Watts. Where the size is greater than 1000 (VA or Watts), the size may be shown as kVA or kW.

The size of the UPS shows the maximum load that can be connected to the outlet sockets. So, if a UPS is rated at 1500VA (or 1.5kVA), the maximum load is 1500VA. A good rule of thumb is to load a UPS to 80% or less to leave some headroom for overloads and potential future expansion.

To calculate the VA load to be connected to the UPS, create a table listing all the equipment to be protected by the uninterruptible power supply. Remember to list every device to be powered as missing a single router or Wi-Fi Access Point can lead to network downtime during a power outage.

Against each item list the Amps or Watts drawn. This information can be gathered from a range of information sources or from a site power audit. Sources of server and networking peripheral power requirements include websites, datasheets, manuals, and even rear rating plates.

The calculated Amps can be multiplied by say 230 (the UK single phase mains voltage is 230V) to give a VA total. This is sometimes referred to as the Apparent Power rating. If the information available is listed in Watts, then the total can be summed to give a total Wattage required for the load. Watts is sometimes referred to as the Real Power rating. For additional headroom and future growth, multiply the VA or Watts by 1.2 to add a 20% safety margin.

A critical point to note is that manufacturers data and rating plate information will certainly be the maximum potential power requirements, which in real life conditions are rarely met. You can choose to oversize the UPS or decide to derate loads calculated from manufacturers data by around 25% or more. Where a more accurate calculation is required, companies like Server Room Environments provide a site power audit service using calibrated test equipment to measure the individual load ratings.

Where you have a mixture of Amps and Watts, for single phase loads, you can either assume that the Watts=VA or derate for a load power factor of 0.7-0.9. Power Factor (pF) is a ratio of the Real Power (Watts) used by the load compared to the Apparent Power (VA) flowing in a circuit. Fortunately, for most small computer and IT loads, the power factor is around 0.9 or unity (1) and that being the case, there can be minor difference between the load in VA or Watts.
Power factor does have a second role in UPS sizing. This is because UPS manufacturers may not rate all their UPS models at unity power factor where VA=Watts. When you look at a UPS datasheet the UPS may be rated at 1500VA but with a power factor of 0.9, 0.8, 0.7 or even 1 (Unity).

The table below shows how the Watt rating for a single 1.5kVA UPS varies due to the power factor applied by a typical UPS manufacturer.

Power Factor VA / Watts Available
1 1500VA / 1500W
0.9 1500VA / 1350W
0.8 1500VA / 1200W
0.7 1500VA / 1050W

From the table, if the load was 1100W and the 1500VA UPS was rated at 0.7pF, then the UPS could only supply 1050W i.e., 50W less than was needed (1100-1050). In this scenario, it would be necessary select the next size UPS which may be a 2000VA (2kVA) UPS which with a 0.7pF rating should provide 1400W i.e., 300W more than is required (1400-1100). As a general guide, most lithium-ion UPS are rated at unity power factor (VA=Watts). Most lead acid battery powered UPS are rated at 0.9 to 0.7. It can be important to check this as it will affect the amount of load and time runtime, the UPS sized to provide.

How Much UPS Battery Runtime Do You Need?

Battery runtime or backup time is the amount of time that the connected battery will support the UPS at a given load (VA/Watts). Whilst it is typically shown in minutes (or hours), the battery set is sized in Ampere-hours (Ah) or Watts pe cell. Ah is a measure of how many Amps a battery can provide in one hour. The alternative to Ampere-hours is Watts/Cell. This measures the maximum power a battery cell can provide during a specific time, until it reaches a specific discharge voltage.

Battery discharge is a non-linear curve. The lower the load placed on a UPS system during a mains power failure, the longer the battery runtime available. This is another reason for sometimes oversizing a UPS. For example, choosing a 2kVA UPS with a 5minute battery to provide 10minutes for a 1kVA load.

Electricity utilities measure downtime in Customer Lost Minutes (CML). Under OFGEM rules, they can face fines for power cuts longer than 3minutes. The utilities are therefore driven to invest in their transmission and distribution infrastructure to prevent dowmtime and reduce power outaged durations. On average, the total downtime experience by UK customers is around 30 minutes per year. For 24/7 IT operations this can impact the choice of battery runtime minutes required, especially as we approach the winter with potential gas shortages which could lead to a greater change of power cuts and planned outages.

More information on customer power outages:

So, for most applications, a short duration battery runtime is required of around 5-10 minutes to allow the UPS to support the load during momentary power outages lasting a few milliseconds to 1-3 minutes. After the power cut, the UPS will automatically recharge its battery, taking approximately 24hours to reach 80% capacity. Longer runtimes of 30m to 1hour may be required if the local site has no standby power generator or the site wants to allow sufficient battery runtime to allow an orderly IT system shutdown if there is a prolonged mains power failure.

Longer runtimes from 2 to 8 hours or more can be specified for a UPS system. For a long battery runtime such as this, the UPS must be sized with sufficient charging capacity to recharge the battery. For a 2kVA load with a 12hour battery, a 6kVA UPS may be selected due to its charging capacity.

The longer the battery runtime, the greater the investment in batteries. The implications here being the physical size and weight of the extended runtime battery and future battery replacement costs.

Adding UPS Battery Extension Packs

Most UPS include a battery set inside their main cabinet and sometimes there is sufficient physical space within the cabinet to add a larger battery set. Where this is not possible the battery is extended using an external battery pack. This may be ‘plug-and-play’ using a DC connector cable or hardwired.

UPS Battery Ageing and Maintenance

Battery performance does deteriorate with age and for lead acid technologies with temperature. The ideal ambient temperature for a lead acid battery is 20-25⁰C. Most server rooms and data centres operate at 18-22⁰C and provide an ideal environment in which to power a lead acid battery UPS. Outside of these environments, temperature may not be as well controlled e.g., basements, small comms rooms and even cupboards. As a rule of thumb, for every 1⁰C rise above 30, lead acid battery design life halves.

A UPS includes automatic battery testing, and this will typically test the battery every 24hours. If there is a fault condition a UPS battery alarm is activated. This can be an LED or audible alarm from the UPS front panel or a remotely reported alarm via an SNMP or signal contact card to some form of UPS monitoring software package or building management system.

For sites with extended battery runtime packs, it can be important to include battery testing as part of an annual UPS maintenance contract. Here a UPS or battery engineer may use a hand-held battery tester to check the impedance and/or ohmic measurements of each individual battery block to compare to the previous year’s recordings or those for a general field population of that specific brand and size of battery. Checking individual battery blocks can help to prevent a costly rapid failure of a battery set, as the performance of a battery is limited by the weakest block in a string.
UPS Site Surveys

For server rooms and data centres, sizing UPS systems can be more complex to ensure business continuity. During a site survey the complete critical power path can be examined in addition to the needs of the IT network to determine any single points of failure. This can lead to additional resilience being built-into the UPS design in the form of UPS maintenance bypasses, automatic transfer switches and parallel/redundant UPS modules.


Whilst we have focused on single phase UPS here, the same principles apply to three phase UPS systems. Single phase UPS systems from 3kVA and below tend to be ‘plug-in’ and therefore easier to install, even with extended runtime battery packs.

Most UPS systems will deliver a battery runtime more than that expected. This is due to a combination of factors as discussed. IT loads may be oversized, 20% safety margins built-in and batteries oversized to cater for deterioration in performance as the battery ages. The acid-test for any installation is to simulate a complete power outage and measure how long the battery runs for. The amount of UPS battery runtime available must be sufficient to ride through expected power outages and long enough to provide sufficient time for an orderly IT network shutdown if required.

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