The US Dept. of Energy has a useful calculator that tells you the cost of producing 1 million BTUs using a variety of heating types and fuel sources. User inputs electric, gas, oil, etc rates. It is at www.eia.doe.gov/neic/experts/heatcalc.xls. The calculator adjusts the rated HSPF of an air-source heat pump based on expected actual performance in the field. See their explanation below.
TECHNICAL NOTE: AIR-SOURCE HEAT PUMPS
The actual heating efficiency and seasonal performance of an air-source heat pump may vary significantly from the rating it receives
when tested under the standard procedures and conditions that manufacturers use to determine heat pump efficiency.
For a detailed discussion of this issue, see the article:
"Climate Impacts on Heating Seasonal Performance Factor (HSPF) and Seasonal Energy Efficiency Ratio (SEER) for Air Source Heat Pumps,"
Fairy, et al, ASHRAE Transactions, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., Atlanta, GA, June 2004.
The method in the form of the regression equations presented in that paper is used in this "calculator" for calculating an adjusted HSPF
for air-source heat pumps. This method is assumed to provide a rough approximation of the actual HSPF that an air-source heat pump
will experience in different locations in the USA.