I think a hot water tank used in that way is generally called a “thermal store”. They work a little differently from a standard hot water tank as you don’t use the water in the store directly, you run it through a heat exchanger into the heating / hot water system instead. I don’t really know anything more about how they are set up or controlled.
The problem with the tank being inline rather than running through a heat exchanger would be that when you don’t have excess solar your boiler would have to heat all the water in the tank as well as all the water in your heating system.