My calculations tell me that propane water heater should be an economical alternative to an electric water heater.
But, looking at Energuide labels of comparable water heater, I seem to be wrong.
- The propane retail price per gallon is $2.60
- The electricy price is $0.16 per kw/h
Both are tankless water heater.
Thermal efficiency is 95% for electricity
Thermal efficiency is 90% for propane
BTUs required to raise 50 gallons of water by 70 deg F
1 gallon of water = 8.33 pounds To raise the temperature of 1 pound of water by 1 deg F = 1 BTU To raise 50 gallons of water by 1 deg F = 50 * 8.33 = 416.5 BTU To raise 50 gallons of water by 70 deg F = 416.5 * 70 = 29,155 BTU
29,155 BTUs per day
Propane gas consumption, per day (50 gal, 70 deg F raise)
1 gallon of propane = 91,000 BTU
91,000 BTU @ 90% thermal efficiency = 91,000 * 0.9 = 81,900 actual BTU per gallon
Daily BTUs to propane gallons = 29,155 BTUs per day / 81,900 BTU per gallon = 0.35 gallon
0.35 gallons per day
Electricity consumption, per day
1 kw/h = 3412 BTUs @ 95% thermal efficiency = 3241 actual BTUs per kw/h Daily BTUs per day in KW = 29,155 BTU / 3241 btu per kw/h @ 95% = 8.99 kw/h
8.99 kw/h per day
Daily Propane cost : 0.35 gallon @ $2.60 per gallon = $0.91
Electricity cost = 8,99 kw/h @ $0.16 = $1.43
It seems to me that propane is clear winner.
Let's change the hypothesis, and say I live in a area where propane is more expensive $3.30 per gallon), and electricity is cheap, ($0.11 kw/h)
Daily propane cost : 0.35 gallon @ $3.00 per gallon = $1.05
Daily electricity cost = 8,99 kw/h @ $0.11 = $0.99
This is an unlikely scenario, and propane is only 5% more expensive
How can energuide rate electricity cheaper than propane?