GSE is pretty easy, especially on new build , the top flashing is a little fiddly to get it sitting nice , a bit of mspolymer/silcon helps

Hi Gusty, if you're thinking of around 4kWp, you could go a bit over it, and use the inverter as the limiter. To be honest, you won't probably miss out on a lot of power when it's over 4kW, cause you're unlikely to have over 4kW of load on a hot summers day. What it will do however is give you more power at all the other times when the sun is less than brilliantly hot, which is a great deal of the time. You're trading a little bit of peak power, for a goodly amount of energy over the year. This would be my preferred option.

If you're thinking a lot more than 4kWp say >7, and you want to be able to use over 4kWp, then you're going to need a DNO, ie. the elect distribution company will have to confirm their system can take the higher current flow (or something like that). That can be straight forward, or it can be a pain - depends on local transformers etc, from what I've been told, or maybe the distributor just wants a contribution to upgrade??

Either way I would say you def should install an immersion export device, which essentially chucks all the energy that would be exported into your immersion heaters, or switch on an elect consuming device. There are no doubt lots of conflicting opinions on this sort of device, that you should be letting the energy export, so your supplier can give you 2p/kWh, and sell it to your neighbour for 15p/kWh. Personally, if you are prepared to take tax payers' money in FIT's, then you should have no issue with making best use of the elect power you have brought to fruition on your roof.

I've had a few different immersion export units installed for various clients on the houses I've built, but being a simple soul, I'm not interested in knowing second by second what's being generated, so I actually installed one in my own house - see link (hope it's still ok to link - I have no connection with this company, or any solar related companies).
http://www.earthwiseproducts.co.uk/shop/

A 4kWp install might be around £5k, but of course local markets vary, and types of panel varies. If space is not at a premium, don't pay more for super high output panels, just have an extra panel or two. If your roof is 12m long, say 11m allowing for edge boundary, and say you could mount 2 rows portrait (11mx3m), that might be 22 panels max. Maybe go for 20 panels, at 250w, so nominally 5kWp. That sort of thing might work for you, letting the inverter limit peak output to 4kWp.

Also think about the number of strings, ie. how many are linked in series - I understand that when they are linked in series, you only get the output maximum of the worst panel. So if you might get a bit of shading from say a stove flue, or tips of a tree, you might want to pay a bit more for individually linked panels, so each one can do it's own thing.

I reckon the current return on your investment is about 6% (17 years), especially if you fit the export utilisation device, plus you have the satisfaction of reducing the grid demand.

Hope something in the above proves useful

In current FIT climates, I think that rather than fit as much, fit what you would use. Look at your base load and how much you could use to heat DHW. Pick your cut off time of year to be 'self sufficient' (excluding base load at night) and size the installation on that.

How does the price vary for a smaller installation?

Many thanks all.