Never mind..... I just noticed you said no natural gas available. Call the utility and see if there are programs, like time of use, or all electric home rates. If the coldest time time (where heat pump is the least efficient) is during off-peak hours, you might get a better monthly cost.
I'll leave it in place as a reference for others however:
With natural gas available:
Your rates are at about $1.00/therm 29.3kWh/therm. Of this, the delivered cost depends on your combustion equipment. Using 90 to 95% boiler, you'll get a delivered rate of
26.37 @ 90% or 3.79c/kWh
27.83 @ 95% or 3.59c/kWh
So, using the average 68 therms and 80% efficiency, that's 5.44 million BTU of delivered heat.
So you'll be using 60.4 therms if you went with 90% equipment.
5.44 million BTUs of delivered heat translates to 1,593kWh.
each kWh at your area costs:
8.3 for first 600kWh
10.3 for portions over 600kWh
so, with first 600kWh probably needed for other stuff anyways and using 10.3cents/kWh, this means you start breaking even on utility cost at heat pump delivered COP of 2.71. (9.25 HSPF equiv).
90% gas:
60.4 therms: $60.40 @ $1/therm
3.5 COP Geothermal
1593kWh/3.5 = 455kWh @ 10c/kWh
Using 68 therm as monthly average given by the utility, and derating to 60.4 for switching to 90% gas equipment if you don't already have one, the 3.5 COP Geothermal over 90% gas system estimates to an whopping $179/year annual savings.
At PSE's rates, payback period is essentially never, forever.
http://pse.com/aboutpse/Rates/Docume...1_panel_pg.pdf