This solar system is designed to use a water heater a backup heat source. You can use a normal tank type electric hot water heater or gas hot water heater as this backup. But, for the most efficient backup - use a tankless water heater or high efficient gas or electric boiler.
The solar tank can have one heat exchanger or two. The bottom heat exchanger takes the solar heat and if used, the top heat exchanger could be use for the back up heater if you have a boiler (This is Optional).
Please note: that when using solar for domestic water only - you can achive close to 100% of your domestic needs with the proper amount of panels (Generally 2 or 3 Solar Panels) and storage. However, you will always need a back up for when there is insufficient solar energy.
All systems are different - so contact one of our solar specialist for a detailed system design for you residential or commercial location.
This is only an example. Actual layouts vary from job to job.