It depends on the rocs payments that you get back as to how efficient the investment is. We installed 4kw in 2015, PV only, no hot water panels. 4kw is the maximum allowed in the north on a domestic set up. The upside is we will get payments for 20 years. This years combined payments are in the region of £700. However this does not take into account any maintainence or replacement panels which may be needed.
My panels have generated 13,900 units we have imported 6450 and exported 11,500 in that period. We get about 4p per unit exported and 15p ? For units we have generated and used ourselves.
The big decider for us was, having downsized from a large house to a small bungalow we had money available to invest. The banks were offering 0.5% . Martin Lewis of money saving expert.com reckoned that installing panels was the equivalent of 7 or 8% return. As an aside because the interest rates were so low we also bought a new Motorhome ! The only new vehicle we have ever owned. No point leaving the money to the kids. It has to be said that the rocs rate of return is adjusted by the government each year.
There is plenty of advice to be had on the net. Look at powerni website ( I know you are in the south) but they do free assessments of likely output and return etc. It has to said they are looking for a roof that sits from south east through to south west. I dare say that you could use the out building subject to the roof being checked for load bearing strength.
Beware the firms who offer to install panels for free giving you free electric. Whilst there may not be problems with the basic model, there are lots of cases were people have had difficulty getting house insurance as the building is now classed as business use. Selling the house can have problems as well since the seller does not have sole title to the property having let their roof to a third party. This was the situation here a few years back. Things may have changed but check.
If I can help further just ask.
Davy
