That was for a 500w pc running absolutely flat out - it won't be and probably won't be anywhere near 500w - link supplied by rogermevans suggests 135w - that would be with a monitor, judging by the rated power on the back of my pc and (15" CRT) monitor it will be less than half that so keeping the sums easy it will be between 50 and 100 watts or between £4.50 and £9.00 a month.
Then again that page is for 2007 so power consumption might be a bit low.
You do realise I am pulling figures out of a hat mostly, even if I am citing sources.
