Not quite the same situation, but I just did a back of the envelope calculation for my house after switching to a heat pump from a 96% efficiency gas furnace.

Comparing last years bills to this year (avg temp was almost exactly the same) I found: Using the therms to kWh equivalence, we used about 1/3 of the energy as last year to heat the house. However, electricity is about 3.8x the cost of gas for the same amount of energy, so my total cost went up some (but not an outrageous amount). I'm willing to pay somewhat more if I'm reducing my carbon footprint, so the next step is to figure out how much NG is used to generate a kWh of electricity. If I'm not reducing carbon emissions enough (or at all) to justify the extra cost, I'll go back to using the gas furnace.