The water in the boiler will keep hot for several hours; the length of time depends on a number of factors:
a. Size of the boiler (water volume): A larger boiler will retain heat longer;
b. Air temperature: The colder the ambient temperature, the faster the loss of heat from the boiler and pipes.
c. Initial water temperature: The warmer the water prior to non-sunny periods, the longer it will retain its heat.
d. Thickness of the insulation: The thicker it is, the longer the water stays hot. In addition, the pipes both outside and inside the building, must be insulated;
e. Usage: If a lot of water is used whilst the the sun is shining, then fresh, cold water flows into boiler and cools down any existing hot water in the tank.
f. Insulation: Thicker insulation will result in reduced heat-loss from the boiler.