You need no more than 3 pieces of information.
First, look at the name plate on the appliance. You are looking for a number with a “W” beside it. This stands for wattage. Some appliances provide the amperage (A) and voltage (V) in place of the wattage. To get wattage, simply multiply amps times volts. An example would be a 120 V heater rated at 12.5 amps. This would equal 1500 watts.
Now to determine kilowatt-hours (kWh) you need to take the length of time (in hours) an appliance operates. Take watts and multiply by the hours of operation. Using our heater example above we would have 1500 watts times the number of hours of use. If we use that heater for 2 hours we would have 3000 watt-hours of electricity used.
To convert watt-hours to kilowatt-hours you must divide by 1000. Therefore, our 1500-watt heater operating for two hours would use 3 kilowatt-hours.