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I have not gone back and done any basic calculations. However, I found an Internet reference indicating 1/5 KWH/gallon to raise temperature from 50 F to 130 F.
If you use 100 gallons per day, then useage is 20 KWH and at $0.10/KWH that would be $2/day. This may not include heat loss from the tank.
In your case put a time clock on your water tank and monitor the on time. If you have a single element heater get a 240 to 120 small transformer. Connect this across the heating element, and on the secondary put an ordinary, non-electronic, clock. Start the clock at 0 (12) and use this a a cumulative on timer. You can also buy electronic and electro-mechanical timers designed for this purpose.
Obviously if you have two heating elements, then one timer on each, and determine the power input to each element.
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