If there’s a power failure and your PC is on, you could lose data. Windows could blue-screen every time you try to boot, especially if you’re running RAID 5 and the power cuts out while you’re writing data to your hard drives. The resulting power surge that occurs when the power comes back on could fry your motherboard, processor, and power supply, as well.

Considering the small fortune you’ve spent building it, why would you be willing to put your PC at risk? Three little letters could save you a whole heap of trouble: U-P-S. An uninterruptible power supply can mean the difference between tears and wondering what everyone else is complaining about.

A typical UPS.

A UPS does exactly what the name implies: it provides uninterrupted power. There are many different types of uninterruptible power supplies, ranging in size from ones that can fit in a 3.5″ hard drive bay to massive mechanical devices that weigh many tonnes.

If you walk into your nearest PC shop, you’ll most likely have the option to choose from an offline (or standby) UPS and a line interactive UPS.

An offline UPS is very basic, providing only surge protection and battery backup, and typically supplies 5-20 minutes of standby power. They usually won’t include a battery monitoring tool or self diagnostic capability. These limitations make them hard to recommend unless you are really on a shoe-string budget.

A line interactive UPS is similar to an offline UPS, but it can also compensate for under and over voltage situations thanks to a variable-voltage autotransformer. An autotransformer is an electrical transformer that can add or subtract powered coils of wire to control the output voltage. Autotransformers can compensate for a wide range of voltages, but due to cost and complexity concerns they are only designed to cover 190 V to 240 V for 220 V power. Anything higher or lower would result in the battery being switched on. If you have to measure the voltage coming out of your wall socket you’ll see that the voltage rarely ever stays at 220 V.

A beeeeg, industrial UPS.

Let’s say that your PC at load consumes 440 watts. That works out to 2 amps at 220 V. If the voltage dropped to 200 V the UPS would draw 2.2 amps to compensate.

So how do you decide what size UPS is right for you? If you have a decent power supply with active PFC, a good rule of thumb is 1:1. So, if you have a 500 watt PSU, 500 KvA should be sufficient. If you have a rather old, or no name brand, power supply with a passive PFC the ratio moves up to 1:1.5. This means a 300 watt power supply would need a 450 KvA UPS. Getting a larger UPS won’t damage your PC, and it will allow you to keep your PC on for longer in the case of a power failure.

A decent UPS is much better than a standard surge protector.

Everyone wants a high-end system, right? Unfortunately, they require tons of power to run, and those systems are consuming more and more energy, thereby increasing the need for things such as coal-derived electricity and fossil fuels. Owners of such systems are also spending a large amount of money per year (which they don’t need to do), and contributing to the global warming phenomenon (which definitely no-one needs to do). So, what if we could save on those energy bills and the planet at the same time? Is there such an option? Yes, by investing in a decent, high-quality PSU.

Anyone want to guess how much power this monster rig would waste on a PSU with a low efficiency?

PSUs are, for the most part, the component we give the least amount of thought to. We buy one that will cover our system’s needs, or that comes bundled with our chassis, and that’s it – we don’t think about it again until it breaks. However, let’s take the average 500w PSU, for example. Let’s say it has 70% efficiency, which is decent for most models. If you max it out to its rated output, your system will run fine and will consume 500 watts of power, but what you don’t see is the power lost by the PSU.

Running at full tilt, it consumes a total of 714 watts from the wall socket. This excess energy is lost as heat, which the PSU expels as hot air. An extra 214 watts is big. Try running that system for an hour a day at full tilt: the PSU consumes, and you pay for, an extra 78,110 kilowatts a year – money that could be better spent on other things.

If we take a higher-end PSU that runs at 80% efficiency, we only waste 125w. Some models even run at 85%, which makes the wastage even smaller at 88w. The benefits of this are many-fold: not only are you saving money, but you’re lowering your entire system’s operating temperature, and at the same time reducing the need for more electricity to be generated to feed your beast.

Efficiency isn’t the only factor: capacitor ageing has to be taken into account as well. If your budget PSU has capacitors that lose 20% efficiency over 2 years, guess what? Your PSU loses as much as 40% efficiency overall, and wastes more and more power as it gets older. When selecting your PSU, not only do you have to take your system requirements into account, but also its power efficiency and the quality of the capacitors inside.

The Vantec Ion2+ 600W: easily one of the cheapest 80Plus-certified PSUs around.

For peace of mind, I recommend that you first assess what wattage your system will consume using an online PSU calculator, and then add an extra 150 watts headroom. A PSU works more efficiently when it’s under less load, and by exploiting this you can bring your system closer to the 80% efficiency mark. As an example, a quad-core Intel with 4GB RAM and a GTX260 should consume under 500w from the wall. By selecting something like the Vantec ioN2+A van-550A 600w, you extend the PSU’s lifespan and save electricity in the process by maximizing its efficiency at those lower levels.

Power supplies: Often the least thought about, yet most important part of your PC. Why is it then, that most people buy the totally wrong PSU for their needs by going too small, or way too large?

Is it really that hard to choose a power supply that’s right for your system? Besides just wattage (which isn’t really that informative anyway), there are other considerations. Should you get a single rail or multi rail PSU? With cable management, or without?  What connectors do you need?

How would you know how much power your PC really needs? That’s the easy part. Simply fill in your specs on the online PSU calculator, add 100 watts for some headroom and Bob’s your mother’s brother. So, for the love of Doc Brown, why would someone who runs a 9600GT, 2.2Ghz C ore 2 Duo and 2 gigs DDR3 ram need a 700watt power supply? Really, now! The number of people who think they need a 1KW PSU is staggering.

You don't need a Mr Fusion to power an SLI setup

Let’s look at a few examples. A basic system, Intel Core 2 Duo E4500, 4GB memory, P35 chipset motherboard, ATI Radeon HD 3650, an optical drive and one hard drive, needs around 90W of power at idle. Even at full load on the graphics card, processor, and optical drive, we still have a total power consumption of only 140W. Now, a mid-level system: AMD Phenom X4 9850 BE, 4GB memory, AMD 790X chipset, ATI Radeon HD 3870X2, an optical drive and two hard drives. At idle, with Cool & Quiet enabled, this system uses 168W, while it needs at most 341W  under full load. A high-end system: Intel Core 2 Extreme QX6850, 4GB memory, Nvidia 780i Chipset, Nvidia GeForce 8800 Ultra SLI, an optical drive and four hard drives. The idle power consumption for this setup is around 310W, and once we place full load on everything power consumption increases to 544W.

As I said earlier, wattage isn’t everything. Like LCD monitor response times and contrast ratios, it doesn’t mean much without context. The most important thing is the number of amps on the 12v rail(s). Combined, anywhere between 30 and 50 amps should be fine for your system. To be honest, if you choose a decent brand PSU, you shouldn’t have to worry too much about amperage. Antec, Seasonic, Zalman, Corsair and Enermax are all great PSU brands.

The Antec Neo He 550 watt is great value for money

Some manufacturers use the peak rating of the PSU when classifying it. For example, the Gigabyte Superb 460 watt can only run at 460 watts for one minute. Its continuous rating is only 390 watts. The 585watt Odin is around 500 watts. Not all Gigabyte power supplies do this though. The Odin GT range is rated for continuous throughput. When you take this into account, it’s easy to see why the Odin GT 550 watt is double the price of the Odin 585 watt.

A decent PSU can be an investment, often outlasting every other component inside your pc.