Awl --
PBS's World Focus had a little ditty on Denmark energy efficiency -- $15 per YEAR heating bills! Holy shit....
ROI on these systems seems to be about 10 years, whose initial cost is about
10% of the house value -- which was either $60,000 or 10% of 60,000 -- heh, just a zero....But inyway, one method was a heat pump/AC that uses buried coils (3 feet underground) as the heat exchanger. I don't know if it's a formal heat pump as in a minisplit ($15 wouldn't go very far, even with inverter technology), or if the underground is just a passive equalizing heat resevoir, with water as the transfer medium.
One home-moaner smartly distinguished "solar heating" from "solar cells", and uses the solar heating for direct heat transfer for hot water, and solar cells/panels (photovoltaics) for electricity -- a separation that allows much bigger bang fer yer photonic buck.
The diff between Denmark and the US in all this is that there seems to be much more government interest, ergo more apparent development and progress -- depending on the PR spin. It's not clear whether this stuff is in the "every man's" home, or still for experimenting arkytecs.
Rainwater is collected in underground tanks, as well, for less critical water usage.
You might be able to catch archives on pbs.org.