Very interesting question UKSBD,as i too am curious about the running costs.
Is yours a lap or tower ?
BTW,sometimes i leave mine ( laptop ) on 24/7.
Skyhi2.
As with most things a common sense approach normally works best.
http://www.maplin.co.uk/module.aspx...energy meter&gclid=CJPlxuOml6ECFdlw5QodqjlLOA
Would these help? Take some readings and use them to calculate how much you use in a week - you then have some evidence of a methodology for the claim.