If you'd like to know how your Power Perfect Box is working for you then use this calculator to estimate your savings. Enter your Kilowatt Hour Total for the month of the previous year. Then enter your cost per kWh.In the next bracket you will insert the number of kWh used in the comparing month. Then insert the cost per kWh in the next bracket.Then below you will see your kWh savings percentage and below that will be your dollar amount savings percentage. .