The yellow "Energy Rating" sticker signifies that the appliances are energy-rated for the amount of power they consume as they are used. This wattage usage is what your utility company uses to charge you for power on your utility bill. Of course, it is rated in kilowatts and you are charged per kilowatt-hour of electricity.
The wattage rating listed on the yellow stickers, for larger apliances, and on the packaging of smaller appliances, can be used to calculate the actual operating cost of each of the appliances, depending upon the amount of usage of each appliance.
Let's say your water heater is a 4,000-watt unit. If you run it for one hour, at 10¢ per kilowatt-hour, the cost would be 40¢ per hour. The math looks like this: 4,000 watts ÷ 1,000 watts (kilowatt) = 4.0 watts. Then 4.0 x 10¢ = 40¢ per hour. Since there 24 hours in a day and approximately 30 days per month, the math would look like this: 40¢ x 24 = $9.60 Then $9.60 x 30 days = $288.00 per month.
This would be true if your electric water heater was running continuously, but it is likely only to run a few times a day when hot water is being used and in high demand. these would include when your family takes showers and baths, while doing laundy, washing dishes, etc... Given that scenario, the math would look like this: 40¢ per hour x 3 hours per day = $1.20 per day. Now take $1.20 per day x 356 in a year = $427.20 per year average usage.
Having said that, I know there will be those of you who will say you use much more or much less than those figures, but I'm demonstrating how to do the math and how these numbers are derived. Hopefully, this sheds a little light on the subject and will help you think and use your hot water wisely and timely to help cut the cost of the electricity that you are using.
Now in the event there is no wattge listed on the appliance, there is no packaging box with the listing available, or the yellow sticker has been damaged where the valuable information once was listed, then you'll need to calculate the wattage using Ohm's law. Simply use an amperage meter to find the amperage draw of the appliance by placing it around the wire or wies feeding the appliance. Once that is found, multiply the amperage and the voltage being used to learn the wattage. Once the wattage is found, use the math above to learn your appliance's energy rating. The math looks like this: Let's say your baseboard heater is connected to a 120-volt connection. The amperage meter shows it draws 8.33 amps. So the math would be 8.33 amps x 120 volts = 1,000 watts. At 10¢ per kilowatt-hour, it would cost 10¢ per hour to operate the baseboard heater. I hope this helps you.