I've had mine a few years now, 15 panels so just under 4Kw. I was lucky enough to get mine done when the FIT (fed in tariff) was 43.3p it's now 13.9p

I bought mine and at a rough guess they save me about £50 a month on electric and generate an income of about £1500 a year, so just over £2000 a year. Initial outlay was about £11000, so I reckon a return on my investment after 6 years, but the FIT is guaranteed for 25 years. Doing a bit better than an ISA