Have decided that the most energy efficient is to revert to an outside toilet with a traditional high level cistern and a pull chain!
Seriously though, that is a point which will reduce some of my energy saved. Hmmmph..... At least the flush volume is still a bit more than what is left behind in the pan, and it rattles through the pan that quickly, there can't be much time for heat transfer above the waterline!
Fortunately, energy saving wasnt the sole reason for doing it! Space saving was the most important one, without having a bog seat that is so close to the wall / cistern that it won't stay up! (Another pet hate! Probably designed by a woman to stop us men leaving the seat up!)