Outdoor unit of mini split variable speed air source heat pump

Cost_per_million_BTUs Comparing Fuel Costs with Cost per Million BTUs

When we consider changing fuel sources to hopefully achieve energy savings, the simplest way to compare the different fuels is to utilize a Cost per Million BTU chart. There are several calculators available on other websites, but a chart is pretty simple to use as well.  We need to use the cost per million BTU as the comparison factor because it is completely independent of your weather or house type. It doesn’t matter if you live in the coldest climate or moderate climate, and it doesn’t matter if you have a new energy-efficient house or an old Victorian house that has no insulation to make this comparison.

To use the chart, you need to know your cost for buying the fuels you’ll be comparing. For electricity we need to know the cost per Kilowatt Hour or KWh. This can be found on your electric bill by adding up all the charges that are “per kwh” which would include such things as “power cost”, “delivery charge”, “renewable energy surcharge” but would NOT include the meter cost or “customer access charge” that you pay just for the meter itself regardless of your use. If you use propane (LP) or heating oil then your cost per gallon, Natural Gas your cost per Therm or MCF.

Next you need to know the efficiency of your heating system and the system you’re proposing as a replacement. For our example, we’ll need to do a little math to get the percentage because we’re looking at an air-source-heat-pump which has the heating function rated in terms of Heating Season Performance Factor (HSPF) but our chart compares it using Coefficient of Performance.


To convert HSPF to COP we need to use this formula:

COP= (HSPF x 1055.1)/3600

Since my heat pump is 12.0 HSPF:

COP=(12 x 1055.1) / 3600 which works out to be  3.5.

This number is the % efficiency and it’s not 3.5%, it’s actually 350%! I know that sounds hard to believe, but the numbers don’t lie and we’re not going to get into a discussion of thermodynamic efficiency, but compared to a regular electric resistance heater which has a COP of 1.0, my heat pump will produce heat for less than 1/3 the cost of an electric resistance heater.

I’d like to compare this heat pump to running my LP Boiler to heat the house, and my boiler is 82% and we pay about $2.00 per gallon for propane.


  • Cost Per Million BTUs for the Heat Pump: $10.46  (our electricity costs about 12.5¢ per kwh)
  • Cost Per Million BTUs for the LP Boiler: $26.66

So clearly the heat pump wins this battle of the BTU’s coming in at about 40% less than running the boiler.