well if they are charging 7.24 cents per kilo watt. u are using 250w which is 0.25kw. its easy to work out i,ll explain so you can work out total extra power (fans etc.). if your using 0.25kw per hour that is a quater of 1kw they will charge you a quarter of 7.24 which is 1.81 cents. so it works...