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Tank less water heaters seem ideal for single people or people who don't use hot water every day, or who don't use a whole lot of hot water, from my figures a 20 pound tank of propane has about 4.5 therms... I think, Natural gas costs me about $.42 per therm or $1.90 for the amount of energy in a propane tank.
Thing is that most on demand water heaters are 60-70% efficient altho the Marely is 54% to 87% (wide range!) efficient and the Vertex water heater is 90% efficient.
My guess is that to make a #20 tank of LP last 100-120 days that you are using about 3 to 5 gallons of hot water per day where I am using about 30 gallons of hot water per day with 4 people in my house.
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