The KW rating is normally provided on an electrical rating plate that is on the water heater. Look for that rating plate to see if it has the information that you are looking for.
If you cannot find the information that you need on the water heater, then the serial number will be needed to determine the KW rating for this water heater. I recommend that you call American Water Heater directly at 1-800-999-5210. Have your serial number and the model number ready. An agent at that number should be able to provide the rating information that you need.
If you need more help from us, reply with additional details (including the serial number) and we will try to assist you further.