If its 2kw then it will use 2 units of electriicty per hour. Depending on your electricity tariff, ie Economy 7 etc will depend on how much it will cost. Your electricity bill will tell you how much each unit is. It could be anything from 6 - 8p per unit.
henrys I think you have made a small mistake. Are you talking about a fan HEATER? (Maybe you live in the Falklands? - we are sweltering here in the UK))
I have a 3-speed 30cm fan and it is rated at only 35 Watts. That means it can be run for over 28 hours at full speed before it uses just ONE unit of electricity.
4getmenot The mistake is either in reading the fan's wattage, or in the description of the item. 2Kw is a HUGE rating for a fan that is only 10" diameter.
If the 2kw is genuine then it must have a combined heating element. If it is a standard domestic fan like we are all using in this heat, then the rating should be around 25 to 35 watts.