On the face of it, it seems like you'd need the coefficient of performance for the electric heater as well, wouldn't you?

Unless the electric heater is considered to have a coefficient of 1, since it looks like the point of the heat pump is to not just create heat (as an electric heater would), but circulate existing heat as well.

If so, you have 1=$2000, and so 2.9=X (which should be less than $2k - I don't know the formulas being used)

The conversion costs would be $15k / ($2k-X), which is the amount of savings per year divided by the cost of the installation.

The savings would = ($2k-X) * Y years (Y being 20 years - the number of years to pay off the conversion)

Keep in mind, it's been a really long time since I did this stuff, and I could be horribly, horribly wrong.