On 2010-02-16, Michael Koblic wrote:
Since Jim Wilkins mentioned Kill-A-Watt meter here some time ago it came on
sale at Lee Valley and I got me one. I spent an interesting hour running
around the house finding out what consumes electricity while plugged in and
what does not. What surprised me were the computers.

My most recent purchase (Compaq Presario) does this:

Turned off 2.1W
Booting 60W
On but quiet 46W
Asleep 5.8W
Hibernating 1.7W

My old computer also draws about 3W while supposedly switched off.

So the questions a
1) Why is the computer drawing any power at all when turned off?

Because it is never truly off. There needs to be enough power
on a couple of pins of the power supply so it can sense the pressing of
the button to tell it to turn on. In the old days, the power switch was
on the side or back of the computer, and truly disconnected it from the
power line, so it was truly off. These days, the systems are set up to
close down gracefully and then drop the power from the drives and the
majority of the system board, so it needs a way to have the system board
tell the power supply when it is truly ready to shut off.

Also -- it probably keeps power to the clock chip to keep it up
to date, and keep the coin cell from discharging -- and to keep from
losing the contents of the setup NVRAM, which is usually a CMOS RAM
backed by the coin cell.

2) Why is the power drawn less when hibernating then when switched off?

That one is a good trick. I don't know -- inefficiences
somewhere, perhaps?

3) What happens to a computer which is turned off and the plug is pulled?

Once the plug is pulled, it can't sense you pressing the power
button, so it can't turn on. If you shut the computer down, unplug the
power cable, push the button once (which would normally turn it on) and
then re-plug the power cable, it probably will not turn on -- unless
there is something remembering that power switch press kept alive by the
clock chip and NVRAM cell.

4) What happens with laptops? Do they draw power from their battery
continuously even when turned off?

There is typically a coin cell buried in the computer to retain
the clock chip and the NVRAM settings. If that goes dead, then it
depends on the main battery. If that goes dead as well, the system will
next wake up with no proper sense of time, and the NVRAM settings lost
as well.

Also, the battery is used to power the switch which turns on the
system, though most of the logic in the computer is CMOS instead of TTL,
so it does not draw significant power when it is not changing state.

But the batteries used in laptops are known for self-discharge
anyway -- the batteries draw enough power from themselves to dwarf what
the NVRAM and switch sensor draw.

Enjoy,
DoN.

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