From day one, a solar hot water system begins to save money by reducing their fuel bills whilst doing your bit for the environment by saving the emission of up to a quarter of a Tonne of CO2 per annum.
High technology evacuated tube panels are most effective on long sunny days, but their advantage is that they will also turn diffused solar energy into solar heated water on brighter overcast days, even in sub-zero temperatures. Direct sunlight simply isn't necessary for this to work.
One 30 tube panel can produce up to 13 kWh of heat gain on sunnier days, providing most if not all of your hot water during the summer months. It will still pre-heat cold water entering the cylinder on many short winter days, and is supplemented by boiler / immersion, but still reducing the amount of fossil-fuel used, saving both money and the climate.