Gavin,how can a shower drawing 20amps be sufficient compared with a 10kw shower?From personal experience I live in a 2014 apartment, the building is made of wood with a bath stone cladding.
the heating and hot water comes from a community heating system, they burn wood pellets on site and pump water to each apartment where a 'boiler' electric powered on a 16amp breaker reheats it.
my heating/hot water costs me between £20-22 a month.
This system apparently reduces the amount of energy lost in transportation and long cable or pipe runs.
I do think though that it wont be heating as we think of it now but will be a mix of solar powered, power wall to store excess and a modern version of storage heaters.
showers will be electric but there are new ones out there that operate at 20 amps which will be more than sufficient for most people. Think of ovens for years we were almost by habit thinking 40 amp and 10mm now im installing ovens on 20 and even 16amp breakers with 2.5 and even had one with a supplied 1.5mm cable.
things are going to change. necessity being the mother of invention and all that.
Regards,S