The energy E in kilowatt-hours (kWh) per day is equal to the power P in watts (W) times number of usage hours per day t divided by 1000 watts per kilowatt:E (kWh/day) = P (W) Ã t (h/day) / 1000 (W/kW).lectricity cost calculation. The electricity cost per day in dollars is equal to the energy consumption E in kWh per day times the energy cost of 1 kWh in cents/kWh divided by 100 cents per dollar: