hi all just questioning how do you work out somethings wattage to how much it will cost per hour to run? i cannot for the life of me remember the workings of this, but am i correct for example 800watt heater run's for 3 hours consumeing 2400watts or 2.4kws, say the price per kwh is 18pence, the heater running for 3 hours would of cost me about 39pence is this correct or not?