Divide the wattage by 1,000 to calculate kW: 1500 watts 1,000 = 1.5 kW. Multiply the kilowatts by the hours of daily use: 1.5 kW X 2 hours = 3 kWh per day. Find the total energy usage for a month (30 days): 3 kWh X 30 days = 90 kWh per month.