[glxxyz]

Yes the figures are my approximate bills so include net metering revenues.

You're right about the 8 year negative IRR for the heatpump, although I'm being very conservative about propane costs, it's likely much shorter. I was pretty conservative about the solar savings too, I generally go for the worst case in these estimates.

Your overall NPV calculation seems a bit off. It's ~21 years to zero NPV at 8% discount rate, spending $80 up front to save $8/year. Factoring in the 10 year interest free government solar loan makes it more like 14 years. My working:

    =nper(8%, -8, 80)
    20.9

    =nper(8%, -8, pv(8%, 10, -4)+40)
    14.3

The solar system is fun to tinker with and should pay off 'eventually', it's not a no brainer of a decision like the heatpump though.

[caminante]

> Your overall NPV calculation seems a bit off.

Correct. It's 21y. I missed $500 from a reading error and was assuming $7.5k/y (not $8k/y).

edit: I see your mention of the grant, too. Combined, that's cutting the NPV=0 point in half from 21y to ~12y. Good job.