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Uninterruptible power supply buying guide

Guide written by:
John, DIYer & IT developer, Brighton

John, DIYer & IT developer, Brighton

59 guides

Offline, on-line and line uninterruptible power supply units are useful for protect domestic or industrial electronic and computer devices, which are as risk when experiencing even momentary power cuts. Our tips for choosing your uninterruptible power supply and protecting your PC and all its data from a power failure!

Important features

  • On-line
  • line
  • Off-line

What is an uninterruptible power supply?

An uninterruptible power supply (UPS) is a protective device for electronic and computer equipment, which by nature are relatively sensitive to interruptions and sudden drops in voltages. If a power failure occurs while the computer is saving to the hard drive, or while your projector's lamp is being cooled by the fan, it can quickly spell disaster. That's why it's always better to properly protect them.

All the while being plugged into an outlet, it contains a battery that it recharges to compensate for any potential power supply failures, as well as converting the DC power from the battery into the AC power required by the connected devices. It also contains a more or less sophisticated electronic mechanism for regulating the voltage in order to filter noise, compensate for voltage deviations or eliminate overvoltages.

Off-line, on-line or in-line UPS?

Off-line UPS 

During normal operation, when the voltage is above a certain threshold, the devices connected to the UPS are fed directly from the main power network. If the voltage falls below this set threshold, the battery takes over and the uninterruptible power supply then transforms the direct current, which it delivers into rough alternating voltage. It's not the beautiful sine curve but rather a square wave. During this transfer of power, there will be a micro-interruption of a few milliseconds (>6 ms).

Devices with power supply equipped with large capacitors are not sensitive to this small break in voltage and are therefore perfectly adapted to an off-line inverter. Despite their inability to compensate for small voltage drops or power cuts, they are inexpensive and suitable in an office setting. Their power can go up to 1kVA.

Line or Line-interactive UPS

Schematically, a line UPS is basically an off-line model with an added voltage regulator that compensates for any input current mishaps and allow power transfer to the battery in less than 2 milliseconds if needed. This regulator, in addition to the protection that it confers, requires less battery and prolongs its life. This type of uninterruptible power supply is particularly effective for protecting sensitive equipment that is connected to a possible unstable grid, experiencing drops or micro interruptions. This situation can be found in emerging countries where in-line UPS units are proving indispensable in the fight again damaging essential and expensive equipment. Less expensive than  their on-line counterpart, their performance far exceeds that of off-line UPS, making it ideal for example, for the protection of small enterprise's IT servers (NAS). They generate a modified sine wave, which is suitable for a wide range of devices. Their power typically ranges from 500 VA to 5 kVA .

On-line UPS

Unlike their off-line counterpart, on-line UPS units continuously generate alternating current from the battery, which is continuously charged from the main electrical network. When the voltage dips to the point of matching the power supply's input, the battery recharges less quickly or is not recharged at all. On the other hand, this results in the output voltage remaining perfectly stable no matter what the conditions. This stability, coupled with an instantenous power transfer makes an on-line UPS  ideal for protecting any amount of sensitive equipment, especially the ones which must remain powered under all circumstances and whose power loss would be catastrophic. Large companies often use this type of uninterruptible power supply for the protection of computer servers, all of which can never fail, at any moment, just like hospitals will protect medical equipment whose use can affect their patient's health status. They are also the undisputed masters of power supply units of more than 5 kVA. They are logically found in many industrial installations or for the protection of circuits supplying numerous and powerful devices.

UPS: protection against electrical failure


Off-line UPS

On-line UPS

In-line UPS





Voltage drop
















How to size your uninterruptible power supply?


Whichever technology ends up being the right choice, the ultimate goal is to allow your equipment to stay powered in case of a momentary power outage. The amount of time that your appliances will continue receiving current despite the outage depends directly on the UPS' power, expressed in Volts Amps (VA). The higher it is, the more time you have to switch off the devices safely. To calculate this, simply add the power of all connected devices in Watts (W), multiply by 1.6 and the result is the minimum power in VA. For example, if you have a PC with a 250 W screen, you need at least a 400 VA UPS, which will give you 4-5 minutes of autonomy to shut down your machine in the event of an outage.

Do not hesitate to invest in a more powerful power supply, if interested in those valuable few minutes of autonomy. It will provide you with more time, and can still adapt to suit any needs, despite the  required extra power.

Which UPS for which hardware

Equipment Type of UP
Generator Line or on-line
Desktop PCs Off-line
Alarm On-line
House On-line
Video projector On-line
Server Line
Home cinema Line

5 tips for choosing your UPS

Choose the correct size for your uninterruptable power supply, based on the devices that you hope to protect, but taking into consideration any future devices you might have!

The price of a UPS may seem high but it is nothing compared to the loss of priceless data or professional equipment.

Powering down a device can also be controlled by the power supply. Thus, those few minutes of added autonomy allows the devices to be safely switched off, even if your absence. It is wise to always check the compatibility between the equipment and the power supply!

An off-line UPS, although very affordable, produces a low-quality voltage that is not suitable for all devices. Check its compatibility with the equipment!

UPSs are sensitive devices , choose a recognized brand that offers a quality warranty.

More Information

For big companies and individuals alike, if you have a need to protect your sensitive equipment, follow the links below for UPS-supply related accessories, advice from our editors and more helpful guides on how to keep everything safe:

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Guide written by:

John, DIYer & IT developer, Brighton 59 guides écrits

John, DIYer & IT developer, Brighton

From a very young age, I was always fascinated by manual and technical work, especially woodworking. When I got my very first flat, it became my own personal DIY playground. I rewired some of the electricity (remember, safety first!) to better supply all my computers and gadgets. I also built partition walls and did some decorating with my wife. We worked on some made-to-measure furniture and came up with little tricks to optimise the space, keeping the original charm of the building in mind. When the little one arrived, I started building bits and pieces for him as well.

We don't have a lot of space, so I don't have a permanent workshop or certain tools I've always dreamed of owning. But with my IT background, I already know a lot about DIY, and I love helping others troubleshoot their ideas!

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