Changing Water Heater from Electric to Propane

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We are in the process of changing the water heater from electric to propane. Renter electric bill has gotten progressively higher over couple of months; we narrowed the problem down to the water heater. Renter feels since their utilities have been higher than normal that they should not have to pay for the first 75 gallons of propane. We could have just changed out the element but decided this would be the best way to lower their utilities. Who is responsible for the first 75 gallons?