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When working out the cost per year to run equipment that is in active use only when you are sat in front of it, we have assumed a usage period of 10 hours a day, five days a week. That means a PC is in active use for 2,600 hours a year, and idle for 6,160 hours.

The figures that we are quoting here are solely for individual pieces of equipment. However, they can easily be scaled up if you want an idea of how much energy an office with 10 PCs is wasting. An increase in the number of power-consuming devices has other implications, though, and they too have a cost.

All electrical equipment generates heat. Anything from light bulbs to PCs and photocopiers emit heat, and the longer each piece of equipment is left running, the more heat it generates. As a result, most office buildings need to be cooled if they are to remain at a comfortable temperature during the day. However, air conditioning is usually turned off at night to save money. If the PCs are left on then that heat is still being generated and will build up overnight, which means that the air conditioning will have much more work to do when it comes back on the following day. This costs businesses even more money.

Desktop PC

Off 3W
Idle 78W
Working Up to 150W
Standby (S1) 59W
Standby (S3) 3W
Typical cost per year £24.53

The desktop PC is perhaps the single greatest offender when it comes to wasting electricity. No-one in their right mind would consider leaving the TV on when

A typical PC draws around 78W, so costs around £22.50 a year to run if it's turned off at the end of each working day. If the computer is left running for 24 hours a day, five days a week, that cost rises to £75.76 a year. The 78W figure is for the PC running a couple of office applications and a web browser. This can rise to 150W when the computer is running games, increasing the cost to run accordingly. These costs don't include running a monitor, which we cover separately on the following page.

If you want to reduce PC running costs, the easiest way is to shut down the computer at the end of each day. It'll still be drawing a little power in this state (around 3W), however, so turn it off at the mains if you want it to draw zero. If you'd prefer not to wait while the computer shuts down and starts up each day, put it in hibernate or standby mode. Our computer drew the same 3W in both states but hibernate, while slower to recover, has the added advantage of ensuring that you don't lose your work if you turn off the power at the mains.

It's also worth setting a PC's hard disks to spin down after a few minutes' inactivity. You'll save around 6W per hard disk that way when the computer is left running. Just remember to close applications that auto-save documents, or webpages that auto-refresh, since they'll make the hard disk spin up each time.

You should also check your PC's BIOS to see which power-saving mode it is using. There are two power-saving modes: S1 and S3. S1 stops the processor while still supplying power to the other components. Our computer still drew 59W in S1 standby mode, which is £40 a year if you put it in standby at the end of each day. S3, however, saves the system state in RAM and cuts the power to most components. Our computer's consumption dropped to just 3W in this state, which would cost just £2 a year.