What happens on cloudy days?
All solar water heating systems come standard with an electrical back-up element which ensures that you have hot water even during overcast and rainy days.
The thermostat is set at the desired temperature, usually 55°. Remember one cannot bath or shower in water hotter than 41-43°. Even on an overcast day, you will be saving money as the sun, no matter how weak, will heat the water to a certain extent, reducing the amount of electricity required to heat the water if it has fallen below the pre-set temperature.