PC using power even when off

[gobbledegeek]

I bought an energy meter recently. You just plug it into a socket then plug in an appliance and it tells you how many watts are being used. Very useful and I've already identified a few 'energy drains'.
However I got a strange reading from my PC. Even when it is switched off (not on standby, but not switched off at the socket) it still draws about 24W. I thought it was because my PC is quite old so I also tested the PC at work and hey presto that uses 24 watts too even when it's switched off! Anyone know what's going on?

[rojash]

Most modern PCs have two On/Of switches: One at the front, and one at the back by the mains input. The one at the front is not actually an on/off switch in the traditional sense. It is a monentary switch (i.e. it only makes a circuit when you are actually pressing it.). This is connected to some circuitry on the motherboard. When you press it it sends a signal to shut down the computer. If the machine is off, it sends a signal to turn it on. Obviously to send a signal, there must already be some power there to monitor the circuit.

When you turn off using this switch, although you are not putting the machine into Standby in the computing sense, you are (believe it or not) putting it into standby in the appliance sense. That is to say, it is rather like turning off your TV using the remote control. The only power being consumed is that used by the monitoring circuit.

If you were to take the cover off the machine, you would see that there is a single LED glowing on the motherboard.

So, the only way to fully turn off the machine is with the switch at the back (after powering down using the front switch or "Shutdown").

Some cheaper power supplies don't have that switch at the back, in which case you must pull the plug!

[rojash]

Incidentally, this is why most BIOSs have an option which allows you to choose what action to take when power is restored after a black-out. They usually offer On, Off, and Last State. Obviously, if there were no power at all to the computer, it could not read the setting and take the appropriate action.

[gobbledegeek]

Thanks Rojash for such a comprehensive answer. I'm still a bit surprised that it's using over 20W just for some circuitry on the motherboard and an LED or two. Also I'd expect some heat to be generated at that rating but can't say that my PC is even slightly warm when it's been off.
BTW I've started to switch the PC and accessories off at the plug since I discovered this.
I'd recommend everyone get themselves an energy meter (try Maplins or Tchibo) - it's quite an eye-opener to see how much electricity some things use when they are on standby (or even when you think they are off!), especially if you calculate the cost per year.

[wildwood]

All the same, 24watts seems to be far too much to keep a computer 'ready for powering up'.

Are you sure the meter is working properly?

[gobbledegeek]

I fairly sure it is working correctly. I used it on a light with a known wattage bulb and it was only 1 watt out. I also tested the PC again today and it was reading 15W this time (having been switched off at the socket overnight). So we are looking at between 15 and 25W.