After a detailed investigation and chatting to many engineers, suppliers, etc I've come to the conclusion that one of the better low-energy technologies for heating water is the Heat Pump. It's widely used in SA by hotels, college residences, mines, hospitals, etc, but hardly known in the domestic sector.
I've found a company in Cape Town that specialises in this - Tekniheat. There are others.
According to them, you can get a domestic heat pump that produces 1400 litres of 60 degree water a day, and which reduces power consumption by 70-80%. Fully installed cost around R18-20K. Apparently you can permanently switch off the heating elements in the geyser(s) - the only power draw is for the compressor/pump, at nominal 1KW. Expected maintenance-free lifespan is 15 years at least, though corrosion can reduce this at the coast.
Anyone here heat their water with a heat pump?