I asked an expert working in the area for a multinational this recently. This was the reply:
Put in underfloor heating, an air to water heatpump, a 6kW solar array and an 8.2kWh battery.
You’d be close to free of energy bills forever. You’d have a small amount to pay in the winter months. Pair it with an electric car of course to make most of the solar and battery.
I think they need to be serviced at least every two years and gas changed. Friend who built a house 2 years ago had the pump fail already. Only a few hundred quid but still.
My question was more of a longer term view early in the process of thinking about a new build in somewhat of a rural location. I haven’t had to put too much thought into the logistics of it yet. It was more what is the optimal solution and shoot for that. And retrofitting isn’t on my radar.