Not actually quite the case.
Lets' Talk PV (Electric)
In the past people filled their roofs as an investment income. (the Feed in Tariff)
Today the systems we install are 'right sized' i.e designed to maximise an ROI and maximise self usage, as such a typically system pays for itself within 11- 12 years and you are paid for 20 years, so rightsizing makes sense (We'll install as many PV systems this year as we did last)
Let's talk Solar Thermal (Hot Water)
It will almost never pay for itself, even with the current government incentives (DomRHI) the pay back (when allowing for a new DHW Cylinder) is about 25 years, the DomRHI will be removed from April next year for solar thermal, so the only system's that we'll install are for either thise that atre doing it as part of a green / sustainable belief or because the architect has included it as party of his SAP calcs.
Having said all that there are now fewer certified installers of Solar Thermal (787) or PV (1883) and only 504 that do both than there were in 2010.