Why do computers use electricty when they are off?

[HFS+]

I thought when computers were turned off they draw no electrictiy. Is this true?

 

[paperwastage]

if you mean physically off (the PSU switch is off), then yes

otherwise, they might still draw electricity (etc your NIC is waiting for a PXE boot command, your BIOS is set to auto-start at 1am)... some of these functions might be powered by the CMOS battery, but some thru the mains

also, some BIOS allows you to set it to auto-start after power loss... how does it know there is a power loss? by probing the PSU/power line for current... energy is required to do this, and energy comes from electricity

 

[Athadeus]

Just to throw a figure in there, all my desktops and laptops (without battery) draw <3 watts while off. My most recent SB 1155 based one registers as 0 watts on my meter, and I haven't bothered to put it on KwH to find out what fraction of a watt it uses.

I think some older machines used to use considerable more power while off. I would test now if I had any left lol.

 

[Jeff7]

Old computers (AT type power supply) had a hard on/off switch - the switch on the front of the PC directly controlled access to the 120VAC power coming in from the wall. With that type, when your PC was off, it consumed no power, since it was no longer electrically connected.

With ATX, you now had soft-off. The motherboard would always receive power from the power supply. Pressing the power button on the front of the computer case now connected a low-voltage signal to a receiver, which would cause the power supply and system to fully power up.
This constant power to the motherboard allowed for other things, such as the network boot feature or timer-based power-on that Paperwastage mentioned. With these things, a chip on the motherboard could be running while the system was not actively running. That chip could then instruct the rest of the system to power up.


More recently, emphasis has been placed on power supply efficiency during low-power modes, so that it's not burning >10W just to provide power to a few small chips that can run on less than a few milliwatts each when idle.

 

[Zap]

It is stand-by power, supplied by the ATX PSU's 5VSB rail.

News flash - your TV also draws power when it is "off." So does your VCR, BD/DVD player, home theater receiver, automatic coffee maker...

 

[VirtualLarry]

What I don't get, is how a PSU can be rated at 2.5A for 5Vsb line, but also "Conform to EUP regulation of <1 W standby".

How is that possible? Look at the EarthWatts 650 at Newegg for an example.

http://www.newegg.com/Product/Produc...rthwatts%20650

 

[pcgeek11]

What I don't get, is how a PSU can be rated at 2.5A for 5Vsb line, but also "Conform to EUP regulation of <1 W standby".

How is that possible? Look at the EarthWatts 650 at Newegg for an example.

http://www.newegg.com/Product/Produc...rthwatts%20650

Just because the 5V line is "Rated" at 2.5 Amperes do not mean it is "drawing" 2.5 amperes while in standby or in running modes. That is just the maximum current draw the PSU is rated to handle.

 

[corkyg]

When a computer is connected to a power source, it is not really "Off," but on standby. Much like your big TV set. If it is connected to a power strip. and that is OFF, or totally disconnected it consumes no electrical energy other than the internal battery keeping CMOS alive.