Thanks for the replies.

When the circuit board that the technicians say is 'sus' is working correctly, we have adequate hot water for our household.

That is the tank lasts us through the peak charge period. It is 290 litres capacity.

Found a Specifications table on the makers web site, and I am sure that for the nine months when our night time temperatures are reasonably moderate [climate is defined as Mediterranean where we live in Victoria, Aust.] the efficiency is pretty good.

So the cheap night rate is the way to go.