It is tricky isn't it. Gas and electric costs are only going to go up. People I have spoken to say that it may take 20 years to cover the costs if costs of gas and electric do not go up too much. However if costs keep on going up, this may perhaps bring it down to 10 years.
Solar will do radiators as well from what I can gather.
Also, the installers can include a solar electric unit to power the pump to send the hot water around.
I have sent for information, but that does not beat speakng to people who have it.
I also gather that in Scotland you can obtain a grant to fund 30% of the cost. Here in England however, you can only claim back