A dumb question but need to know..... 2

The 300 Watt figure was the maximum load of the power supply, and refers to the sum of the partial loads of the 3.3, 5 and 12-Volt phases.

You would need to determine exactly what was active during this overnight period, and add their power requirements together. This chart will help:
Note that things like the floppy drive, mouse, keyboard, display .etc are likely not active and therefore their power requirements would be zero. Some attention should be given to the power saving features of the PC. See the options under Control Panel, Power Options.

A call to your local electricity company might be worthwhile, I am sure they would have statistics about power costs and consumption. You may also find that the power cost charges are reduced between certain hours overnight. It may even be the case that the actual powering up of the machine consumes the same amount of power as does running it for an hour or so?

Personally and from a safety point of view, I prefer to turn off a machine (and unplug it) overnight if it is not in use.

Show it to him in black and white, er, black and green, ah, digitally:

[navy]Freestone[/navy] Thanks for that. I did a bit of googling and found these for those of us in the UK, two devices, one of which measures the whole house and the reading is via a wireless hand held! So you can go around turning things off and being astonished!

[navy]linney[/navy] You raise an interesting point. I think it may be worthwhile to see what people think about turning off versus leaving on. So I have started a new thread on the subject. Here