Electricity here is €0,30-0,40/kWh after tax. (Netherlands) (For fixed contact, dynamic contacts have bigger variation). That makes it way more interesting to save on energy. Although getting solar panels is still the fastest investment. Heat pumps typically have 10-15years payback time, especially now they are insanely overpriced due to Ukrainian-Russian war and climate goals increasing demand and lowering supply.

so when electricity costs drop by 15–20× the payback time will be 150–300 years?

The temperature difference most home heaters work on is much higher than the one most home coolers work on.

Using a regenerator to get 5°C of cooling back with some 30% of efficiency probably won't be cost-effectice. But if you have to heat by some 30°C, the numbers change a lot.

hmm, are you saying that the regenerator (or recuperator) has lower efficiency at lower ΔT? or lower efficiency when it's more a ΔH than a ΔT? because if that 5° of cooling and dehumidification costs you the same US$100 a month as the 30° of heating, saving 80% of it will still be US$80 a month

Winter has much less sunlight, so powering a heat pump via solar panels + moderate battery becomes much harder. It's no longer mostly free.