Solar thermal is great for bulk low-grade heat but as soon as you want higher output temperatures (DHW rather than space heating, heat pump input or DHW pre-warming) in less than ideal sunlight conditions PV begins to look attractive. As you say, the flexibility to use electricity for applications where hot water is less useful (running computers, etc) also pushes you in the direction of PV.

My calculator for comparison of PV and solar thermal: http://edavies.me.uk/2012/11/pv-dhw/

(Note, the price of the PV panel used there has gone up a bit since, but not enough to swing the basic conclusion.)

Re: Thermo dynamic panels, they are just ASHP without a large fan. I like the small size of them, the quite running and have been told that they have been used for years in Portugal.
What I cannot find out is any reliability data about them and out of the two installers I have spoken to, one refused to get back to me, the other did not fill me with any confidence (just like their windows).


When comparing PV and ST you have to me careful what you are comparing. kWh alone is not much use. It is a good unit to describe PV with as you know (assuming grid tied) that the voltage is 230V (ish). With ST your equivalent to volts is temp, so if your store is cold, your kWh, even in the winter, can look impressive, but you may not get hot enough to be useful and requiring supplementary heating. If your store is hot, your ST adds nothing, so your kWh figure is zero. So you need to look at performance above a set temperature, say 45°C.
As Ed describes on his website, there is a small 'sweet spot' where the two are about even, with the balance in performance and cost being in favour of PV.
It really comes down to how much water you need heated every day and what your weather regime is. Both are variables. PV does not need servicing (though component may need replacing).