Re: What?
"Double glazing and good insulation has the best return."
Couple that with a solar powered (mains backup) GSHP for your HVAC needs and you're in the handful of bucks per month territory for the life of the product. My setup is about 15 years old and shows no signs of failing ... although I'll admit the solar is a little long in the tooth, and the battery bank needs a second rebuild. Plan is to swap 'em out for LiFePO4 within the next year or so, the rest will be fine for our needs for a few more years (decades?).