This makes a lot of sense:

Hot water systems are important and are one of the most basic energy storage devices available. Before, electric hot water systems were timed to charge at night, but recent observations show that it is better to have them charge in the middle of the day when excess solar energy is being exported to the grid. 

Source: How Hot Water Systems Can Be a Great Alternative to Solar Batteries

Whole house solar batteries are expensive. Instead, energy could be stored in other ways, such as the hot water stored in a large hot water tank. By sizing the tank and doing its water heating during the day, the tank is effectively storing the solar PV energy in the form of heat.

Another is to use a small tank and a small, separate PV system to pre-heat water from the ground temperature (say 55 deg F) up to 80 degrees, before entering the main water heater tank, where it then gets lifted by another 40 degrees. That idea makes sense in some situations, but not all.