Let me give you my experience of living with solar PV for the past 6 months.
I've downloaded the lifetime statistics from the inverter via Bluetooth and I can see that my 4 kW array has produced an average of 9 kWh per day since May 2012. It faces South East so the majority of the electricity is produced in the morning.
This year, I will receive 25.9p per kWh generated and just over 3p per kWh for exporting it, of which I only receive 1.5p per kWh because it's argued that I use 50% of it.
The upshot is that £900 a year is paid up.
I believe that the previous owners installed them for around £8k. So a 9 year ROI is about right.
However, people installing a solar PV array now, will not get the same rates as me. The Conservative government have been reducing the rates for new installations since 2011. I have an excel spreadsheet detailing all the various rates and it's quite confusing but today's installers will get less than 10p per kWh.(pitiful when you buy electricity at 20-35p per kWh)
You really do need a South facing roof, to maximize the chance of capturing sun. A cloudy morning for me and a lovely summer's evening is lost sunshine. Likewise a SW facing roof misses the morning sun.
Another fact from experience is that Sunny days do produce more electricity than cloudy days. A good June day can produce 24 kWh, gloomy December day around 3 kWh.
Below is a graph showing the energy curve of a typically sunny day. Ignore that break at 9:30, Dan!'s electrician was hooking up the ASHP. The peak is around 3.5 kW during summer and I've yet to see what winter bring but I reckon 3-5 kWh days will be the norm. If only I could compliment it with a small wind generator without affecting my FiT.
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I passed two closed stations and then queued for about 30 seconds to be able to buy all the diesel I wanted (for the Volvo).
