On 1 July, 21:23, "tim...." wrote:
I guess that this is off topic but I suspect that I will find some experts
here.

I keep my expert in a book:
http://www.withouthotair.com/

David MacKay (and if you're a geek, yes it's the same David MacKay)
has written what is by far the best guide to energy policy and
alternative generation approaches. Sums too. It's even there as a free
download. This book is very highly recommended.

My sister has had a man around to quote for this and I've been tasked with
checking out if what she was told makes sense.

Time to break out Excel and do some real modelling of it. For this
money, you can even afford to buy a bottle or two of something for
someone who does understand financial modelling with Excel.

There are broadly four issues he

* Does the tech work, and does it last? Read MacKay, but the stuff is
now pretty good and pretty predictable.

* Does the sun work? Site surveying isn't the black art some make out
and you can make good predictions. Some years will be better than
others, but over a few years' lifecycle, it's good enough. Read MacKay
(and some of his further reading - try ther Navitron website too)

* What price will you get? This cannot be justifiable for "real
cost"(sic) electricity. However the massive porkbarrel of the feed in
tariff does make it close enough that you have to check the numbers to
know which side, you can't simply assume. However what's the political
future of inflated feed-in tariffs? (IMHO it's good - it's so small a
market that the cost to Whitehall is tiny and the press reaction to
cutting it would be bad).

* What does money cost? It's a big capital outlay. However if you're
like many of the bungalow owners I know, there's capital available, no
bank interest to do much else with it, and a wish to create a return
on it.

The rest is numbers and you have to do this with care. However don't
decide something like this (risking a large investment) based on an
estimate, calculate three estimates (mean, worst, best) and do your
real decision making on the worst case assumpitions and your risk
assessment of how likely this is to happen.

There's also the question of getting a second supply installed, as a
source of electricity - because you won't want to use any that you
could have sold for 3x market price instead! However in practice, you
don't use that much domestically when the sun's out, unless you're at
home during the day.