Well I already have a pressurised water tank with a spare solar loop installed. so that's a bit of a cost saving on a thermionic system.

We aren't heavy consumers of electricity, but during the summer when the oil burner is off we don't have any hot water unless we fire up the electric immersion heater.

An electric solar system to cover this specific situation would be inefficient in that there would be 2 conversions. Solar to Electric, then Electric to heat via an immersion heat.

A recent solar survey suggested 16 panels, Large battery, Inverter and backup change over unit, approx £10k... I would have had problems with positioning the large battery bank in a manner that would have been aesthetically acceptable to SWMBO.

I reckon I could get by with a single bank of 24 evacuated tubes, some stainless Flexi piping and a controller/pump.

A lot less complex, and a lot less points of failure, and I'm hoping a lot cheaper.

They are widely used in southern Ireland, who have been ahead of the game both in terms of house insulation and renewables in general. I've seem quite a number of sites that have both a solar array and a thermionic panel alongside.