Possible causes of the problem you mentioned:
1. Heavy scaling in the collector, especially in the vacuum tube, forming thermal resistance and affecting heat collection;
2 . The hot water tank is not well insulated and dissipates too much heat. You can touch the outer wall of the water tank at night if it is not hot, this problem can be eliminated.
3. Use plenty of water during the day.
Actually, it is not a good thing if the temperature of the solar water heater is too high (above 70 degrees)