I am building a new house. I can put a 105 gal. electric hot water heater on the off peak electric meter - it would only gets juice from 11PM to 7AM.
My other option is to use a LP water heater.
How can I compare energy costs between these two units? The Rheem 105 gal heater is about $1100 dollars. A 40 gal LP is around $500
These are not comparable units, so it is difficult to compare them. If you are trying to compare the cost of fuel, then you compare the price of LP in your area on a per BTU delivered energy basis against the cost of electricity. In your case, this is a bit difficult, since you apparently are considering running the water heater only at night. At 105 gallons, there is a reasonable chance you will not run out of hot water during the day. Figure a shower at about 2.5 gpm, a load of wash at about 10 gallons hot water, normal hand washing etc. another 15 gallons per day per person, and you can estimate based on how many people are in your house whether you can go all day without running the electricity.
At 40 gallons, the LP heater is almost certainly going to run several times during the day. You can still compare the cost of fuel on a direct basis, only thing to be careful about is that electric heat is effectively 100 percent efficient, while you will be lucky to get 70 percent efficiency out of the LP system, so take that into account when calculating the cost per BTU for fuel.
For a more realistic comparison, compare a 100 gallon LP to a 100 gallon electric, or maybe a 50 gallon unit would make more sense. You pay a lot for the 100 gallon size, and probably cannot make up any cost difference based on fuel alone.
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