James Hunt examines a piece of kit that could save you a whole lot of trouble if the worst happens
If you've ever accidentally kicked the plug socket or tripped over the power cable of your PC, you'll probably know that a sudden unexpected power-down can have dramatic consequences - particularly when it comes to hard drives, which can be accidentally wiped or actively damaged by a sudden power cut.
Of course, even if your PC isn't damaged by unplanned power loss, it can throw other spanners into the works. Maybe a long process you've been running for days is lost, or maybe your plan to remotely access your system while away is thwarted. Whether it's a circuit breaker being tripped, a wider power cut or human error, losing power to your system is always inconvenient. And if you want to prevent it, an uninterruptible power supply is the way to do it.
What Is A UPS?
When used domestically or in offices, uninterruptible power supplies are designed to provide continuous power when the mains supply becomes unavailable. Rather than generating their own power, UPS devices contain batteries, which are charged during normal use and then begin to discharge if the power supply is cut off. In a computing context, they're usually aimed at businesses, where server stability is crucial to providing continuous operation, but in recent years they've become affordable and compact enough to have in the home.
Domestic uninterruptable power supplies work on the basis that most power interruptions don't last for very long, and they only support a useful power output for a few minutes. This does give you the ability to either restore power without losing any of your work or safely power down your system should the interruption last longer than average.
It should be noted that they aren't intended to replace a mains supply for extended periods. UPS devices only store a limited amount of power, so it's important to decide what devices to run off one and to be aware of the power they draw. Attach too many devices, and the UPS will discharge its stored power too quickly, making it ineffective for its intended purpose.
The power rating of the UPS will be given in both volt amps and watts, so you need to take account for the power draw needs of your system (or systems). Ensuring the UPS has 25% of additional capacity over your immediate requirements will give it plenty of room to charge while delivering power to the devices it supplies, and give it room to lose capacity due to natural aging of components. UPS devices for industrial and commercial use may be capable of powering entire buildings, but the average domestic UPS is rated at just a few hundred volt-amperes, which is enough to power a single computer system for a few minutes.
Although the primary function of a UPS is to provide continuous power, the way they work also means that they're able to smooth out surges, sags and spikes to provide a 'dean' (meaning consistent) power supply.
The result of this is that systems are protected from surges that could damage components and interrupt normal usage. Indeed, if your system is overclocked you might be particularly vulnerable to power surges, because overclocked CPUs require a very clean power supply to maintain their finely tuned operation.
Choosing A UPS
UPS devices can be deployed in a large number of contexts, which means the market has a huge range. Broadly speaking, the market can be divided up into 'tower' units (which are stand-alone, self-contained devices) and 'Rackmount' units, which are customisable devices designed to sit alongside server PCs in racks. This isn’t a particularly good distinction, however, as a tower unit might describe a small under-desk device designed to keep a single system in operation or a unit the size of a large fridge-freezer intended to support an entire office.
If you're buying a UPS for domestic purposes, you're most likely looking at a low-end tower unit, which will cost anything from £50 to £500. Before you make a decision, you have to decide what your power requirements are, so you can ensure you buy a UPS that can provide enough power for your needs and that can provide that power for long enough to be useful.
When calculating your power draw, it's worth remembering that the system itself will have peripherals necessary for use. Unless it's ’headless', you'll also need to provide power for your monitor and maybe devices like a router or modem if you're keen on protecting these devices from power interruptions as well.
Calculating the power requirements of every device is tricky (you can use an online wattage calculator to give you a rough example) but we'd recommend that you start by simply checking the wattage rating of your devices and add them together. A 500W PSU isn't going to pull 500 watts all the time, but calculating in this manner gives you an upper ceiling for your UPS requirements and ensures that you have room to continue running your system even under the most taxing conditions. Too much battery life is better than too little, after all!
If that seems like playing it too safe, you can use a smart meter or other monitoring device to check the power draw in real time and use that as your basis. Remember to account for all the peripherals you want to run - put them all on the same multiplug and then test the power draw on that -and test your PC when it's running a complex operation, like a game or HD video, so you aren't just checking the power draw at rest. This will hopefully give you a more accurate reading, which you can use to better allocate your budget.
Once you know what the power draw of your system is, you have to calculate the volt-ampere load, since this is the unit most UPS devices quote their capacity in. Multiply the combined wattage value of your system and its peripherals by 1.6 and you'll get the minimum volt-ampere value your UPS needs to supply. For example, a 400VV system would therefore need a UPS capable of outputting at least 640VA.
As well as identifying the device class and VA output, you'll need to determine the type of technology the UPS uses. There are three types, and knowing which you're looking at will help you account for some potentially large differences in the pricing of otherwise similar-looking UPS models.
The cheapest versions are known as 'offline' or 'standby UPS devices. If you try to buy a UPS and it doesn't claim to be anything other kind, that's because it's this - the most basic type.
Offline UPS units simply charge their batteries and then switch to them if the power supply is interrupted. The switch takes only a few seconds, which is easily within the tolerance boundaries of most electronics, so there should be no ill effects from the miniscule interruption it causes.
Line-interactive UPS units work in much the same way, but include a transformer that can regulate power during brown-outs and sags, so the power doesn't have to go off completely for the UPS to make itself useful - any drop in power is compensated for. Line-interactive units are slightly more expensive than offline models, but not especially so.
Online UPS units are the most expensive and require a significant amount of extra technology inside them. Unlike the other types, an online unit completely isolates any devices it runs from the mains supply and handles the flow of power at all times. Any device plugged into it effectively runs off the battery, and the battery itself is constantly recharged while mains power is available. This is particularly good if you have very sensitive hardware to run, as any loss in mains power is not accompanied by any interruption, but it does come at a significant price premium of anywhere from 200 to 400 percent.
For obvious reasons, we recommend that home users restrict themselves to an offline or line-interactive unit depending on their personal needs, but unless you know you need an online unit, you shouldn't for a home PC.
UPS Efficiency
Calculating how long a UPS will run your system before once the power goes off isn't an easy job, which is why manufacturers often give the estimated run time for a half-load (i.e. drawing 50% of the available wattage). Unfortunately, that’s not particularly useful in real terms, and that's above the usual embellishments manufacturers stack on top of their devices. For a start, a half-load isn't actually the mid-point of the maximum time a UPS can power a device for. A full load runs for about 40% of the time a half-load runs for, so if a device with 600W output runs at 300W for six minutes, it'll run at 600W for about 2.4 minutes.
Unfortunately, calculating the performance of a UPS yourself is quite difficult. You need several pieces of information to get started: the voltage-ampere rating of the UPS device, the number of battery cells it houses, the DV voltage rating of the batteries, the device's efficiency and the capacity of the batteries in ampere-hours. A lot of this information is only available in the manual and not in online spec sheets, so you may have to do a bit of digging to find it.
Once you do have that information, though, you simply multiply the battery voltage by the AH rating by the efficiency, then divide the result by the VA rating. This should give you a relatively small number, which is the estimated power delivery time in ampere-hours. You can then multiply that number by 60 to get the result in minutes, and then that value by the number of cells to tell you how long it can run in total.
Although it's a useful exercise in understanding how UPS power requirements are calculated, it's often not very practical to actually perform this calculation for every device you want to buy. Rather, we suggest trying to find reviews of any UPS you want to buy and relying on their assessments.
Other Features
If type and output don't help you pick a UPS, you'll want to consider what other features the unit offers. We've already touched on a few things UPS devices can do that makes them more than a simple battery-backup, but let's look at them in detail.
First of all, you'll want to consider the software compatibility of the unit. Most modern UPS devices have a feedback ability that allows you to monitor the efficiency and usage statistics over a USB connection. This can also be used to provide a notification to the PC itself to the point of performing an automatic safe shutdown if the battery levels get too low. When you buy a UPS, you should look for this feature and check that it's compatible with your OS, particularly if you're running Linux or Mac OS X.
If you don't want to use a feedback connection and software control panel (or can't, because you have a nonstandard OS, for example), then you might want to go for a UPS that has an on-board LCD display. While most devices have rudimentary indicators in the form of LEDs or beep codes, the more advanced information available in a software control panel is usually replicated on an LCD display where available.
It's also worth checking the number of sockets the UPS offers. Some have only one or two sockets, some have eight or more, but it's important to note that not every socket will necessarily be connected to the battery backup. Some UPS units give you off-battery sockets that won't be powered if the mains supply is disconnected, but they may still have surge protection features. Likewise, many offer surge-protected and filtered telephony and network points for your modem or Ethernet connections.
Fans of noiseless computing will want to check whether their UPS has a fan inside as well. Large units usually contain one, but smaller ones don't. If you go for a unit class that requires a fan, then the same rules as ever apply: larger fans are quieter because they run slower.
Finally, it may not be an immediate concern, but you should check whether the batteries are replaceable. Although they're based on slightly more sophisticated technology than a laptop battery, UPS batteries still deteriorate over time, and most have a useful lifespan of about three to five years. Swappable batteries can be replaced by the user, but sealed units will have to be replaced entirely. Don't expect cheap units to have swappable batteries, though - the price of the battery itself is usually between £50 to £100, so it wouldn't be economical for the cheapest devices to support replacements!
And that concludes this guide. Hopefully you now know what a UPS is and how to go about buying one. All you need to do is decide how much use it would be for you!