Replies

1. Expert Member
   BILL WICHERS | Dec 09, 2020 05:15pm | #1

   All you need to know to do this is a few conversions as follows:
   1- 3,412 BTU per kilowatt hour of electricity
   2- 91,500 BTU per gallon of propane
   along with your pricing per kilowatt hour of electricity (make sure you add in distribution and connection charges. The easiest way to do this is to convert your entire bill to a kw/h number by dividing the total amount of a bill in dollars by the total amount of kw/h used during that billing period), and your price per gallon of propane.

   Now you need to know how efficient your heating device is. For gas-fired burners, you can just read the tag. An "80% efficient" unit gives you 80% of the total fuel BTU content as useful heat. This means it will use 1.25 gallons of propane to give you that same 91,500 BTU as USEFUL heat -- the rest is wasted heating up the exhaust gasses, basically.

   Electric heat can be more interesting to calculate. If you are using electric RESISTANCE heat (baseboards, space heaters, etc.), then it's easy -- 100% efficiency, you get out the exact amount of heat as the total energy you put in. This means 1kw/h of electricity gives you the full 3,412 BTU of useful heat.

   If you have minisplits, things are much more complicated since you have to know how much useful BTU output you'll get per kw/h at whatever efficiency the unit runs at with a given outdoor ambient temperature.

   Assuming you have electric resistance heat and an 80% efficient propane fired furnace to compare, and you only want a RELATIVE cost difference (which means fancy stuff like degree days for your climate zone don't apply), this is how you run the calculation:

   (BTU per gallon of propane * furnace efficiency%) / (3,412 BTU per kilowatt hour)
   (91,500 BTU/gallon * 80% efficient furnace) / (3,412 BTU/kwh)
   (73,200 useful BTU per gallon of propane) / (3,412 BTU/kwh)

   21.454 kw/h is equivalent to 1 gallon of propane (73200/3412 = 21.454)

   Let's assume you pay $2/gallon for propane and a total of 15 cents per kilowatt hour of electricity. You can convert in either direction, so I'll just convert propane to electric for the purposes of our comparison:
   $2 / 21.454 kwh/gallon = 9.322 cents (equivalent)

   This means your cost to heat with propane would equal your cost to heat with electricity if your electric rate, after all fees, was 9.322 cents per kwh. If you electric rate is higher than that, propane is cheaper. If your electric rate is lower than that, then electricity is cheaper. In the case of my example, propane is a bit shy of 40% cheaper than electricity for heating.

   Since this is a ratiometric comparison, the absolute % savings is the same regardless of how much your heat runs. If you spent $1000 over an entire heating season for electricity to run your heat, propane would have cost about $622 for the same heating season, for example.

   If you want to work out what your costs are LIKELY to be REGARDLESS of fuel type, things are much more complicated and you need to know about heating degree days for your area, and a bunch of info about your home's likely energy consumption which will involve some educated guesses.

   Bill

1. 
   arnoldk | Dec 10, 2020 07:59am | #2

   Thank you Bill. I'll give it go.

   Arnold