Worst ones on standby are computers. These can use between 3 and 10KW when in "sleep" mode (and 60-300W when in use). 10kw divided by 1000 = 0.1kw/hour X 28p/kwh=2.8p/hour. Left on standby for, say 18 hours, this would cost 50.4p/day or £15.33/month and lower 3kw PC would be about £4.60/month. Obviously varies depending on level of kit you have in your set up but might be worth considering for any gamers on here.