We are building in southeast North Carolina and would like to know if there is a table which shows at what temperature it becomes more economical to run a LPG furnace instead of a heat pump? I am aware there are many variables which could affect this table but a general idea would be appreciated.
Thanks for the help,
BobS
Replies
Bob, have you called your local electric company? I have found them to be helpful in advising the break point from one fuel to the next. Also good to know for setting the crossover point for backup heatpumps. They will ask what efficiency furnace and same for the HP in order to make the comparison. Without knowing the fuel costs or efficiencies, I would guess that a season of heating with the HP will be cheaper than using LPG, but would be interested in what you find out. If you go with LPG, I would buy my own tank, and not be tied to one supplier. Paul
Yeah, I think the electric company will generally have a chart or formula that you can use. Plug in the cost of gas and electric, efficiency of both units, etc, and get a result.
But basically it's pretty simple: You figure the number of BTUs you get per dollar with gas by factoring together the cost per gallon, BTUs per gallon, and furnace efficiency. You figure the number of BTUs you get per dollar of electricity by factoring together the cost per KWH, BTUs per KWH, and pump efficiency (greater than 100%, of course) for a presumed outside temp. As you vary the outside temp, the pump efficiency will vary, and that will in turn vary the BTUs per dollar of electricity.
One problem you'll run into, though, is finding a chart of pump efficiencies by temperature. They tend to lump it all into one relatively meaningless number to make things "easier".
Here is a cheesy little quick and dirty spreadsheet i did for my kid's house. Get you HP data off the web and inset the numbers.. The values on the ss are for Puget sound energy in WA state, will be differten for Duke power?? It often never gets below freezing here, so it would not even pay to consider a LPG tank here.
One caveat, the ss does NOT take defrost cycles into account, so the elec. cost is probably about 5% higher.