Let's get the general stuff out of the way before going on to specific kinds of panel.
There's plenty energy from the sun hitting the surface of the earth. If you could usefully capture all the solar energy striking your roof, it might well supply your entire energy needs. But that's a big 'if'! Because (a) you probably can't capture anywhere near all of it, and (b) most of it arrives in summer when you don't necessarily want it. At our latitude, 52 degrees north, there's about 10 times more energy available on an average day in mid-summer than in mid-winter.
There are two very different kinds of solar panel that you might put on your roof. I'll include
the links to my particular pages on them here, but please read the rest of this page on
more general stuff first.
N.B. The Low Carbon Buildings Programme grants were cancelled in May 2010, so the information below remains as just some notes on the way things used to be. With the introduction of Feed-in-tariffs from April 2010, and the possibility of a "Renewable Heat Incentive" from 2011, then make sure that your potential installer, and the product they wish to install, are on the registered list to qualify. For some silly reason, the schemes insist on this, so do-it-yourself unfortunately doesn't qualify.
There used to be grants towards installation of these technologies available via the government's Low Carbon Buildings Programme. Until early 2007, it was possible to obtain grants of up to 50% of the cost, with no upper limit. Unfortunately these turned out to be far too popular for the remarkably small pot of money, with the result that it used to run out within minutes of being made available at the start of each month. Pity I wasn't in the market for a system back then!
So the scheme was first suspended, and then resumed later in 2007 on much less favourable terms, leaving anyone trying to set up a business installing these technologies never knowing how many customers they were likely to attract from one month to the next. A complete mess. Who knows how long the present grant scheme will last.
From late 2007, one could obtain up to £400 towards a solar thermal system, and up to £2500 towards a solar photovoltaic. £2500 is also the overall limit even if you get both, or indeed other technologies not mentioned here. You have to satisfy criteria including having insulation and low-energy lights first, and you have to use a registered installer, and install a registered product. To my mind, the registration scheme is a bit of a scandal too - businesses are charged quite a lot each year to be on the list - maybe OK for the huge companies, but not so good for small traders. And too bad if you feel up to doing a DIY installation, which isn't particularly difficult - you then can't have a grant, and have to pay full VAT on the materials.
My own opinion is that the £400 towards solar thermal wasn't worth the bother. You'll probably end up having to use someone who charges more in the first place. But by all means investigate it. But £2500 towards solar photovoltaic makes more sense - more of the cost here is for the material rather than labour, so prices don't vary so much.
At the time I got my panels (late 2007 and early 2008) the planning situation was confused. Each Council seemed to treat solar panels differently, as the regulations weren't designed to accommodate them. Rather annoyingly, South Cambs. District Council insisted that I obtain planning permission, even though, with different interpretation of the rules, they could have decided that the panels were 'Permitted Development'. Not surprisingly, the permission was granted without problems, but this of course took time, and wasted several hundred pounds of my money.
Since then, the Government has passed two amendments to the General Permitted Development Order, each of which increased the circumstances under which solar panels would be 'Permitted Development' and not require an application or explicit permission. It's probably still as well to ask the planners, but I interpret the rules in early October 2008 as meaning that most solar panels on roofs, regardless of whether or not they face the street, and even in Conservation Areas, do not require planning permission (as usual, Listed Buildings still require a special application). Do email me if you want pointers to the legislation.
I've not yet come across this with solar photovoltaic installations, but unfortunately, the world of solar thermal installation seems to have become infested with people similar the those who used to give double-glazing salesmen a bad name.
You might have seen leaflets with ludicrous claims. Solar thermal provides hot water. It won't provide your central heating, even slightly. At least not unless it's an enormous system which would then produce so much excess heat in summer that it would be difficult to know what to do with it.
And a solar thermal system should cost something starting from (say) £1000 DIY, or £3000 fully installed. It may be more, within reason, depending on how large it is, and what extra work is needed.
If you find the leaflet or web-page doesn't give the slightest indication of price, then be suspicious. If you can't find out what the price is likely to be without a 'salesman' calling, then take care. I've heard the usual tales of salesmen who won't leave, who quote silly prices (say £15000 or more), but have the usual gamut of 'deals' - a slightly less but still ridiculous price if you sign immediately, phone calls to the 'manager' to get 'special (rip-off) deals', etc.
Here are the links to the details of the two kinds of panel, and my own installations: