@Whiternoise: I am quite sure that 100 years is over-estimating it too, I am not going to argue that point, but even if you halve it, or take a quarter of it - that's still more or less the lifespan of those panels, and likely beyond the warranty you would get if anything goes wrong.
You didn't suggest they would be sufficient for all the toys, but I was trying to figure out why you think it would add to convenience. If it's not going to make you mostly self-sufficient, then surely it can only be yet another potential thing to go wrong? Unless I am missing something obvious here.
I won't argue with the +£5000 points. I've not looked up, so I'll take your word on it. Still, if you are going to take the trouble of building a house, I'd expect it to be for keep. Anyway, I -do- like the idea of using renewable energy, but as you've said efficiency is not particularly great right now. If I was to go down this route, I would be worried that a couple years down the line, we would see significant advances in both efficiency and cost/efficiency that I may regret not holding back a few years.