I have been using a solar thermal water heater for nearly two years, and am now ready to share my experience with it.
First: solar thermal water heaters (often just called solar water heaters) operate by absorbing sunlight into insulated (by vacuum) glass tubes where it turns into heat. That heat accumulates in the water until it becomes adequately hot. The hot water is stored in a tank.
Apart from that, the heater was said to be equipped with a built-in auxiliary electric heater to ensure that hot water is always available, regardless of weather or time. That electric heating element was never plugged in, and the heater has been providing hot water even on cloudy days!
This is made possible by the simple, yet amazing technology of insulation. The heater heats the water during the day, and insulation keeps the water hot throughout the day.
It is a 53 gallon rooftop unit with a retail price of approximately $1,100, including installation, but that was reduced even further by a discount.
It paid for itself in one year because it reduced the monthly electricity cost by $50. That is what the previous 1980s heater cost.