Isn't this just like saying you'll be able to sell them to the person you sell your house to for half the price you paid for them?

LOL.

I think it's more that the economic value of solar is about half of what you pay for it. That is determined by how much you save on your bill and how much value you place on being 'green', compared to the Jones'. The increase in property value would only represent that value, not what you originally paid for it.

Yes, but that is true of everything.  I don't live in the UK, so I don't know how common it is for houses to have pools there, but here in California it is quite common.  You'd think that a pool would add to the value of a house, but if someone doesn't want a pool they might actually ask for a price reduction because they don't want it (although probably only in a buyer's market would you see much of that).  Home values are always about what accessories you add are worth to other people, and it's almost never full price, even if it's something they want.