What happens when the sun is not shining?

Update Time:2018-05-21
In general, a solar hot water heating system should be able to generate up to 85% of your hot water needs from solar energy. In the event that there are many cloudy days in a row, or you have increased demand in your hot water consumption (several guests spending the weekend at your house), a back-up element kicks in and provides the required amount of hot water.