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The energy rating of an appliance tells you how efficient it is. You may see stickers that have a tower of letters with the more efficient ones labeled A, A+, and less efficient ones as C or D. They tell you how much of the energy you supply is used usefully. For example, you may supply a kettle with 100J of energy of which 15J is used to heat up the water. The other 85J may be used as sound or for heating the surroundings in other words wasted. Another kettle might also be supplied 100J from which it might use 90J to heat up the water. As you can see the latter uses the energy more efficiently and so will have a higher energy rating. Energy rating can also be thought of as power since power tells you the rate of energy transfer.
You can use this formula to estimate an appliance's energy use: (Wattage × Hours Used Per Day) ÷ 1000 = Daily Kilowatt-hour (kWh) consumption 1 kilowatt (kW) = 1,000 Watts Multiply this by the number of days you use the appliance during the year for the annual consumption. You can then calculate the annual cost to run an appliance by multiplying the kWh per year by your local utility's rate per kWh consumed.