Everything in a PC draws its juice from one metal box sitting in the corner of your case. While you busy yourself with graphics cards and processors and delve into overclocking, it just sits there humming away.

Unless it packs in, or you realise you don't have enough connectors after buying a new component, you probably never give it much thought.

The basis of the PSU is the transformer, first conceived by those brain boxes Joseph Henry and Michael Faraday in 1831. Without the transformer we'd be in real trouble because the mains electricity that comes out of the wall does so at a sizzling 240 volts, which is great for running the vacuum cleaner and electric fire, but useless for transistor radios. It needs taming, transforming, rectifying and smoothing.

There are only two reasons anyone would buy a new PSU. Either your existing one has stopped working and smells funny, or you're finally building your über machine with half an eye on world domination. Either way, you want something efficient, quiet, and capable of running everything with room for expansion.

A soft benefit of investing in a new PSU is the better power efficiency offered by new models. The rise in awareness of power consumption brought on by the eco movement has meant that the overall transforming efficiency of a power supply has become news, with stars and awards for being good. You can pay anything from £13 to well over £200 for a PC power supply.

Over the next few pages we'll tell you what you can expect to get for your money, as well as answer some common questions on the subject. How many watts do you need? Does the quality of the power supply matter much? And what exactly does it do and how?

We've put some of the market-leading power supplies through their paces to see who delivers what they promise, and whether expensive electronics add much to the mix. But first, you'll need to know a little more about your box of power.

There are two main power supply formats: AT and ATX. Since ATX has been around since 1996 we'll skip over AT power supplies, which are strictly for retro systems. The ATX standard has now reached version 2.3.

The main motherboard connection is a 20 or 24-pin block, which carries all the voltages. The extra four pins your PSU may have supply more power for motherboards that need it. Many PSUs have cables with the four additional pins on a separate block so you can use old motherboards – it's a handy feature that you may not know you need at the time of purchase, but certainly worth looking out for.

Processors get their own 12V supply, which used to be via a 4-pin plug (often called a P4 plug), but more commonly now via an 8-pin plug (the EPS12V). Like with the motherboard power cable, you'll often find PSUs that split the 8-pin block into two for compatibility with old processors.

Modern CPUs don't run on 12V, however. Motherboards have on-board voltage regulators to lower the voltage to the right figure (the excess being given off as heat). If it's any good at its job then your graphics card is the biggest draw on power. The PCIe slot can only deliver 75W (that's actually a fair amount for any component in your rig, but not for a graphics card).

This quickly proved inadequate, and so extra 12V cables were drafted in. Initially, these had a 6-pin connector, and added another 75W. Later, 8-pin connectors were introduced, which delivered 150W of extra power. This is not to be confused with the 8-pin EPS12V plug for the motherboard, which is wired differently. If you have to push and shove really hard to get either one in place then you've plugged in the wrong one.

You'll want lots of PCIe power connectors so you can swap and upgrade cards and allow for dual card setups. You might well need four 6-pin connectors or more in the future.

Why run so many 12V lines? The voltage drop rises as the current rises, so it's better to have more low power lines than tax a single one. Plus, with the amount of power some of these graphics cards draw, a single wire would quickly turn into a heating element.

Then you have SATA power cables, which carry all three voltages. You'll want at least three of these, but preferably more. You'll also find old-school 4-pin Molex peripheral connectors for IDE drives, and possibly a floppy disk drive connector and some 'intelligent' fan connectors.

Most decent PSUs use modular connectors, so the power supply has a set of sockets rather than permanently connected wires. This allows you to connect only the cables you need, so you don't get a mess of wires hanging out of the unit, blocking airflow.

Some sources say these modular designs are less efficient, as the extra block connector introduces resistance, and hence waste. Apparently, it's measurable, although we've yet to see any tangible evidence. It's probably true, but completely irrelevant. The extra convenience and flexibility of a modular design outweighs any tiny loss (and it will be really tiny).

Think inside the box

So what's inside a PSU? If you're expecting a big soft-iron-core transformer and not much else then you're in for a shock. Modern PSUs are switched-mode power supplies, which use switching regulators to flip between full on and full off at very high frequencies (50KHz and up). This minimises waste, and means they can be much smaller and lighter than full-on linear supplies.

A PSU supplies different voltages, and has separate circuitry to deliver each – these are the rails. An ATX power supply has a 12V, 5.5V and 3.3V rail, plus an additional rail to supply stand-by power. However, this wasn't enough, so ATX 2.0 added a second 12V rail to help power those graphics cards.

The first rail, 12V1, is used to power the processor, and the second rail, 12V2, powers everything else. It's this second 12V rail that's going to take much of the strain of your system.

Some power supplies boast more 12V rails, which is great, but there's no advantage other than the higher overall output. In fact, virtually all supplies split one 12V single rail into two, each with separate current-limit circuitry. This avoids potentially dangerous levels of electricity (240AV is the specified maximum, which is 20A per rail). However, you can't draw as much overall power through both lines because they share the transformer.

Fully independent 12V rails are expensive, although they give a cleaner signal. The 5V current is usually taken off the same transformer, and the 3.3V current is created by voltage regulators. In the past most power was required at the lower voltages, so the 5V rail took most of the load. Times have changed though, and now it's the 12V supply that's doing most of the work, with the additional 3.3V rail for the newer, low-voltage components.

The different power distribution between rails is one reason older power supplies don't always work on newer motherboards, despite being able to deliver the watts on paper. What your PC would really like is a lovely, flat DC signal, sending 12V right across the line. However, the PSU has a job to do here because the mains is a rather dirty 50Hz AC current. You can't just chuck a transformer and a diode at it and expect a smooth, constant voltage.

This is where the smoothing capacitors come into play. These little beauties can be the difference between a quality PSU and a poor one.

Waste not, want not

Power supplies get hot, and this heat is, of course, wasted energy. The ATX 2.3 specification requires an efficiency of 70 per cent, and recommends one of 80 per cent. That's still a fair amount of loss – your 400W PSU is drawing 500W at the wall.

There are higher efficiencies available, and there's a whole green movement to promote them. However, try not to get too carried away with it and spend a fortune on a 95 per cent efficient supply. Electricity is still relatively cheap, and you probably won't see any large return for your outlay. We recommend models on the '80 Plus' list or any model that's Energy Star 4.0-rated.

Another factor you should consider is the loading. Like most machinery, a power supply has sweet spots where it's most efficient. You need to avoid the extremes. You wouldn't drive from London to Manchester at an average speed of 30mph. Nor would you make the same trip at 150mph (even if you could get away with it). Sure, it's possible to drive at such speeds, but you'll burn through fuel much quicker than if you cruised at the optimum speed.

It's the same with PSUs: you should aim for a load of between 50 and 80 per cent – anything more or less will introduce waste.

Temperature plays a part too. The colder it is, the more output you get. In an ideal world, PSU power ratings should be accompanied by a temperature. If this temperature is below typical operating values (30-50 degrees), you know you've lost a few more watts.

What else might you consider? Well, there's the noise from the fan, and of course, its looks. In terms of sound, you don't want a constant reminder that your machine is on. As for looks, rugged and handsome, preferably with lights, always goes down well (although you'll lose your green cred with these).

Is more power better? Not necessarily. Some spare capacity can always come in handy in the future, but going wildly off spec adds nothing. There are an awful lot of high-power PSUs out there running well below capacity. It's tempting to show off with a high headline power capacity, but it won't increase your PC's speed one jot. Go for a high-quality and efficient supply, rather than the big number.

Now, the most important question: am I going to run two really powerful graphic cards at some point? This is the one component that really draws the watts, and you'll need to be prepared for it, or spend the same money all over again on a slightly bigger PSU.