Solar panels - I have the vacuum tube type and have been really pleased with them since they were finally installed correctly. This took the firm nearly a year, but they have since gone out of business, which is hardly surprising. Most years I have not needed to turn my boiler on at all during the summer - sadly not quite so good this summer.
In the first year that I had them they saved me about £300 on the previous year's bill, and I am sure that this has gone up pro-rato every year since - but not yet this year.
My PV panels I have had for several years now. I was warned at the time that my installation (only 7 panels were possible because of the siting of the thermal panels on my roof) was "barely economic" - but I am very happy with them.
They are producing about a 9% return on my capital investment, which was a small private pension lump sum.
66 years old living on my own so cutting costs is a big issue with me, but I have been prepared to invest in the technology to help me do this and to put into practice efficient ways of making the best of them (easier with the solar thermal water I find).
It's easy for the hot water - don't run the hot tap to wash your hands at any time of the day - if you wash-up by hand soak all your crocks until the morning and wash up then so that the sun has the best chance to reheat the water.
If possible also shower in the morning, but if you need to shower at night then at least it will be as hot as possible from not having been used before.