An electrical appliance of 2500W is turned on for 2 hours. Power is the rate of consumption of energy per second. One watt is one joule consumed in a second. 1W = 1 J/s.
Electricity consumption is specified in terms of kW*h. An appliance with a power rating of 2500 W or 2.5 kW consumes 2500*2 = 5 kWh in two hours. As the cost of this electricity consumption is $1, the cost of 1 kWh is 1/5 = $0.2
The unit cost of electricity is $0.2 / kWh