I like the idea there of using Rasberry PI, or something similar. But I do think Solar is an interesting idea.
Ok, the cost of setting up a half way decent power station can add up, but if you are using it for other things as well as mining, it will spread the cost.
There is another interesting facet to this power rating thing. A Power Supply Unit may well be rated at say 1300w, but there is no way that it draws that much power continuously. Most miners as I understand it get hot, dissipating the power being used, but you would need a wall sized heat sink, and AC Cooling to stop a full 1300 watts from frying your whole room. That's a 1 bar radiator .... So has anyone actually put a smart meter on their rig and watched the throughput. I'd be really interested in the results. You can get those things that plug into the household circuit now that tell you how much power you are using. If you have a Smart Meter in the UK, it's brilliant. Power and Cost. Right there on a digital display.
Anyway, Solar may not be the answer in the UK ... but solar + wind may be. A basic setup like that is about £350. Scale it up from there. Batteries - big ones, deep cycle type to hold over the nights and no-wind/no-light days...
Of course, Just buy Bitcoins, but where's the fun in that :-) If I was interested in storing up ... wealth ... I'd buy gold and stash it under the bed.  But I'm not interested really in Bitcoins as a source of income, and projecting out a ROI over 10 years would be nuts. It's all going to change/go away/stop long before then. And I'd still have my solar setup. It's like putting solar power on your house, and you just work off that. Ok, nothing going into the grid probably, but it's still pretty cheap power because you are running your house on it as well.