Solar hot water is weather dependant, to get daily legionella free solar hot water you have to install a solar hot water system that incorporates weather patterns  and the four seasons in its design. That is a hot or warm summer season, autumn and spring being less warm  and winter being cooler.  To have solar hot water through winter you need a larger collector, and be able to deal with excess heat in summer. Weather patterns for example a few days of sunshine then a few days of cloudy weather. You need a storage system designed to heat up quickly and have a storage capacity to carry through cloudy days.