Question: how do you calculate electricity usage?
Answer: Watts (electric usage rating of your items, 1 kilowatt is 1000 watts) ... 
Watts/1000 = kilowatts or kW. ... 
Hours used in month (use an average) ... 
Kilowatts x Hours = Kilowatt Hours or kWh. ... 
Electricity price (in pence per kWh - enter the cost you pay)