If you want to fin out how much the pc uses in one night / day, its simple maths. Pick a start time - ie 07.00 and go look at the leccy meter. Record the reading right down to the kWh - electricity is measured in kWh. Then - do what ever you do during the day- best if out at work and no one home. Then, as you get back in - read the meter again. This is having had the pc on all day.
Following day, same experiment - this time with you out and pc off all day. Read meter at start and when you get in.
as you will have used more - albeit slightly on the day its on, take the evening reading from the morning reading and this will give you the amount of kiloWatt hours used.
Do the same for the day the pc is off.
Say now that the day the pc was on and nothing happened but normal stuff during the day, look at that figure- for example- you used 12 units (kWh).
The day it was all off, you used 6 units (kWh).
Now look at your recent leccy bill and find out how much 1 unit (kwh) cost - ie 0.06 pence- therefore, on the day it was on the property used 12 x 0.06 = 0.72 cents or pence.
Following day with pc off and average day, you used 8 units - again 8 x 0.06 = 0.48 cents or pence.
Over the period your pc was off, 0.72 - 0.48 - saved you 24 pence / cents
Thats the way to calculate usage.
As someone that works on these things - they dont use a lot of Â£Â£Â£ the first thing the voltage does is drop down in a pc from here in the uk to 110v from 240. Then it just gets lower due to the electronics in them
Always turn pc off and disconnect from wall in an electric storm.
Hope I have helped
ex fire engineer and knows a few things about electrics - now what were those colors - red to red, green to green = blue-= to bits!!!