Solar is simple. In some homes your existing tank may be simply configured to accept a solar panel system.
In most homes a new tank is installed to replace your existing tank. This is because some home owners do not perform regular maintenance on their existing tank, such as, flushing out sediment twice annually or using de-lime in areas where they have poor water conditions. So just in case (the previous owner) didn't take care of the tank, we can install a new one.
In Caliornia most homes will benefit from an "Active" or "Open Loop" solar water heating system. This means that cold water is pulled out of your tank and passed through your solar panel on your roof and is heated and pushed back into your tank.
The pump that pushes the water to the roof through panel is traditionally operated by a small electric generating solar panel so you are using no additional electricity to run your solar heater. Additionally, in the event of a power outage, your solar system will operate as normal.
Because the solar heated water is much hotter than the thermostatic setting in your tank the element remains off, meaning you are saving gas.