This is a simple thing to do.
When cold water is drawn into your hotwater tank as you use it it is obviously cool or cold water. So you need to heat it up to temp from the cool point.
SOOOOOO what you do is get an old hot water heater that does not leak. Tear off the outside shell and insulation on the tank. Set it up outside where it will get the most sunlight during the day.
1]Paint the tank black.
reroute the cold line that leads into the tank you use to heat the water to the outside tank cold inlet.
The outside tank hot outlet will then be connected to the cold inlet on the inside tank. Connect any safety features to the outside tank. blowoff valves etc.
No electric current needed for outside tank
the outside tank is solar heated. So the water in the outside tank will be brought up to a higher temp. The higher the temp of the water on the outside tank is that many degrees you will not need to heat with your energy cost on the inside tank.
Temp zones will determine how many months of the year you can use the outside tank.
Ever go out in the summer and pick up a water hose and started spraying water and found how hot or warm the water is that is in the hose that has been exposed to outside solar rays and air temp.