Kilowatt hours and energy cost
Energy in kilowatt hours is power in kilowatts times hours, and the cost is that figure times the price per kilowatt hour.
A worked example
A 900 W dishwasher run 20 hours a month uses 0.900 x 20 = 18 kWh, costing 18 x 9.0 = 162 cents at 9.0 cents per kWh.
The mistake people actually make
Convert watts to kilowatts by dividing by 1 000 before multiplying by hours. A kilowatt hour is an amount of energy, while a kilowatt is a rate.