There's only one thing you've got wrong there, if you're tank is boosted to 70 through night it's beyond where the efficiency is for solar top up. Most of the energy is used in the first 30 deg of temp rise this is where you find the best efficiency gains.
That's interesting to know, thanks
During the summer, the immersion heater rarely gets a daytime boost from solar because the 'stat doesn't cut in.
But it's there if we have hoards of grandchildren invading us and having long hot showers

But what we often do when exporting surplus to the grid, on most sunny days, is heat up the old hot tub.
It only has a small 2kW heater and it's nice to know that we are getting it heated for 'free'

Fortunately, SWMBO does not want to use the hot tub in the winter
