By far the most lucrative outcome of solar PV is the FIT, which for us pays 48p per kw/hr - around 3 times the cost from the grid. Savings in consumption alone will take forever to pay back installation costs. We are careful not to use the washing machine or dishwasher outside of daylight hours, and not simultaneously, to optimise use of PV generated electricity.
Solar thermal will definitely not be as lucrative as PV for us but installation costs are less and we now have equal hot/cold pressure/flow. The shower is far stringer and does not stop or go cold when a tap is opened, as it did when using the combi boiler. Further savings can be made by switching to hot + cold fill to washing machines and dishwashers as water heating by electricity is where most of the running costs are for these appliances.
I suspect the calculators used to estimate savings from solar thermal err on the conservative side and by taking the above steps, along with frequent data monitoring, further savings are available.
Ask your installer to mount data devices where you can easily see them otherwise you won't take the trouble to check them and respond to what they show.