This is such an interesting subject, and one I care about immensely. Efficiency is like free money.

I could put in a full A/C system in a flat my size for roughly €10k. The time and difficulties in regulatory compliance are not included, and significant. The cost for electricity here in Europe is high, VERY high since the Russian invasion of Ukraine. Instead of A/C we built a pergola with a lot of plants. I designed and installed a watering and misting system with control valves and timers. All for about 1/3 of the cost.** This has turned (slowly, plants take time) our upstairs terrace from an open kiln in the summer to an oasis.

Our need for central A/C has gone away for the foreseeable future. The money we didn't spend on a central A/C installation is nice, but the savings on running costs for electricity are enormous and never-ending. Just as importantly, our home looks really nice with the pergola AND is much cooler. We are working on a similar plan for the downstairs, with the added addition of solar panels over the pergola (more time and a LOT more regulations). That money we don't spend during the summer months running the A/C adds up quickly.

With the increasing number of sunny days in Vienna, the increasing efficiency of solar panels, and the increasing cost of electricity, mid- to long-term we'll be at or near net-zero energy.

One last point that is not covered by Mr. Lovins comments. Retrofitting apartments and condos is not nearly so simple as new builds or retrofitting single dwelling houses. Possible and cost-effective, yes. Simple, no.

** We did buy a window A/C unit (included in that 1/3 price) which we use during the (ever increasing number of) days over 35°C that we have each summer. My wife and I would have been OK, but we got a very young and traumatized Husky from a shelter in the spring, and he was suffering massively in the summer. Dogs are clever, but not clever enough to understand that running up and down the stairs looking for a cool place to lay down only heats you up. I have many photos of him laying directly in front of the A/C.