Home Office UPS Buyers Guide

Home Office UPS

A UPS designed for a home office environment is typically only going to protect a handful of devices - a computer (desktop, laptop or workstation), single or dual monitors and potentially an external storage device such as a NAS. In this home scenario, the UPS is primarily there to provide a short window of uptime should the mains power fail so any data can continue to be saved without issues, applications can be closed properly and devices can be powered off as normal.

A typical UPS intended for home use, sometimes referred to as an offline or standby UPS, are normally free-standing tower units designed to sit on the floor. It will usually have regular UK 3-pin mains plug type connections, so computers, monitors and peripherals can simply connect the UPS and the UPS will then connect to the mains wall socket. Many will also have the ability to support and protect phone, network, serial port and data connections. Other features often include a battery replacement indicator, battery management and intelligent features such as automatic save and shutdown.

An offline or standby UPS provides light surge protection and battery back-up for the connected devices. Once it senses that the main power source goes beyond acceptable limits or fails, it will switch to the “offline or standby” battery. As the power source physically switches, there will be a small transfer time between the main power source and battery - usually around 25 milliseconds, so you won’t notice any downtime. It is also worth mentioning that there may be noise from the UPS when running in battery mode so placement may be a consideration, although the nature of this should be minimal.

UPS Sizing

There are several factors that influence sizing a UPS system, including the combined load of all the equipment the UPS will protect, scope for further system expansion, battery runtime and redundancy. Correctly sizing an uninterruptible power supply is crucial as it will not power all the connected devices in the event of a failure if the battery capacity is too small.

The first step is to calculate the total power range for the combined critical load that needs protecting. The power consumption of electrical equipment is stated in either Watts (W) or Volt-Amps (VA), and equipment labels and supporting technical manuals will provide information on volts, amps and overall power consumption. Because UPS systems are rated by VA or kVA ratings, this may require a conversion from W to VA.

1. List all equipment to be protected by the UPS - remember to include monitors, external hard drives, routers, etc.
2. List the amps and volts for each device. Then multiply amps by volts to determine Volt-Amps (VA). Some devices may list their power requirements in watts. To convert watts to VA, divide the watts by the power factor. For most devices, the power factor is 0.9.
3. Add the VA values from all the devices together.
4. Then multiply this by a figure such as 1.2 or 1.25, which factors in future growth and system expansion. That figure is the minimum size in VA or kVA that your UPS should be. Note that a UPS should never be sized to run at 100% load capacity, as this isn’t recommended for safe, stable and reliable performance.

Lets look at an example from our diagram above:


NAS - 30W
PC - 200W
Monitor 1 - 25W
Monitor 2 - 25W


30 x 0.9 = 27VA
200 x 0.9 = 180VA
25 x 0.9 = 22.5VA
25 x 0.9 = 22.5VA


Total VA = 252VA
252 x 1.25 = 315 A

Suitable UPS - 350VA

This example shows that a 350VA rated UPS would be more than adequate for this home office set-up allowing expansion room for extra devices, whilst running the UPS at around 75-80% of its capability.

To avoid doing the calculations manually there are online calculators provided by the leading UPS manufacturers, that either let you input the wattage figures if you have them, or select devices from a list provided. Click here to access the APC calculator.

Battery Runtime

This is the amount of time you want the UPS to keep equipment operating in the event of a power failure. This depends on the nature of the equipment, but in a home office scenario, runtime only needs to be for a few minutes allowing work to be saved and devices to be shut down properly. Typically a home office UPS will have a single battery installed that is replaceable but not expandable or upgradeable - so you need to make sure you have sufficient battery runtime to perform the tasks you wish to.

Using the example above a small 350 VA rated UPS may provide enough power capacity to protect the NAS, PC and two monitors, but would only allow one minute of battery runtime. To gain more runtime you would need to select a larger UPS (perhaps a 650 VA rating) - you may only use 50% of its capability rather than 75% but the resulting runtime will be greater - several minutes allowing you to ensure things shut down without issue. Most UPS brands will provide a runtime graph, like the one below, for each model showing the expected runtime as the load increases.

Graphs like this are particularly useful if you have spare capacity on your UPS and want to know how the runtime will be affected by adding more devices or a more power hungry PC. It is worth mentioning that if you have a NAS device or media server in your home office, then a longer runtime will be useful - this type of device may be constantly saving or backing up data and sudden interruption may cause data loss. Having ample battery time to be able to shut them down properly is invaluable in protecting your data.

Mission Critical Home Office

Although this section of our UPS guide is aimed at the home office, there may be occasions where a home environment has either multiple computers running or where sensitive data is being used and its integrity is key. In these cases it may be worth considering higher end UPS models offering greater VA ratings, longer runtimes and features such as advanced automatic shutdown, back-up integration and scheduling software. It will usually mean that you will need to change your device power leads to IEC (kettle lead style) connectors rather than standard 3-pin plugs, as UPS models aimed at businesses use this type of power interface. However this may be a small price to pay for the addition feature sets and extra runtime. To understand more about higher-end UPS models, read our Business UPS section.

Time to Choose

We hope you’ve found this guide to selecting a UPS for a home office useful. You can now click the below link to explore our great range of UPS devices.


If you would still like some advice on sizing or choosing a UPS, don’t hesitate to contact our friendly advisors on 01204 474747.