So it's mostly an air-source heat pump with a bit of solar gain to it ? When you need it most (if it's your primary heat source) it's be at it's lowest efficiency - mid winter, freezing outside, b***er all sun, CoP drops to the point where it's better to just use direct electric heating. When you are getting solar gain, you are still using the heat pump to get usable heat out from it (it looks like a poor design for a solar collector) - but at least your CoP should be reasonable (do they ever get above 3-4 ?)
After a quick look, I assume you are referring to this system :
http://thermasol.ie/thermasol-thermodynamics.html
It's very telling that the "data sheet" gives f***-all useful data with which to actually evaluate it's true running costs - as opposed to the marketers claims. They only give one table of figures with power in and power out, which evaluates to a CoP of 4.3 (smaller unit) or 4.7 (larger unit). They don't give any parameters for these which I assume are when conditions are most favourable.
Given that here, lecky is 4 times the price of gas (last time I looked), a CoP of 4.3 means you'd be better of with a gas boiler once you take into account the cost of the system. I very much doubt you'll get a CoP of 4.3 most of the time - like when the sun is down, it's cold outside, and there's a few inches of ice on the panel !
it also depends on what you already have. If you're doing a new build or refit and like either UFH or oversized rads then that's OK, but for a retrofit you'd need to factor in the cost of converting the heating to run at low temperature. Oops, they don't mention heating, so you'll still need your gas boiler for that - 2.8kW isn't going to do much for you unless it's a very small property.
I suppose if you don't have mains gas then the figures may be a bit more favourable. But I'd want to see some actual figures, not the marketing department's idea of what comprises a technical data sheet !
