Just a thought for those thinking about installing solarwaterheaters in temperate/northern climes. Even when the sun is not obviously shining, solar collectors are still absorbing radiation. This may (on cloudy/cold days) be only sufficient to pre-heat water, BUT, this is still worthwhile in that pre-heating water reduces energy consumption costs considerably.My costing figures for the 2012 weather washout in the UK show my solar installation as an efficient investment. If I was solely relying on conventional heating sources, the 2012 costs could have been eye-watering.
When people/neighbours question me about my use of alternative energy generation sources, I always tell them that the primary installation of a solar water heating array was my best decision for seeing a tangible return on investment; combined with reducing carbon footprint. Heating water is an expensive business, and the solar array has reduced costs (& carbon) considerably.
PV panels have been installed secondarily as an offset for electrical energy consumption. Next thing will be a wind generator, possibly.
However, if a person has only one choice for reducing energy consumption & carbon footprint, I would suggest that the solar water heating array is the primary choice. Other technologies can be added later.