The things in our household where we use heat and cold are among the things that soak up the most electricity, so this includes our washer and dryer, our refrigerator, and of course, the water heater.
To go completely solar for your water heater needs has some definite disadvantages:
1) A typical system can heat about 50 gallons of water per day. If you have more than two people living in your household, or take a lot of showers, you will need a backup conventional water heater.
2) Typical system purchase and installation costs run about $5000 - $6000, and it will take a long time for the system to pay for itself with the money you save on your utility bill
3) With that kind of cash, you can buy all new energy efficient appliances (Energy Star program) and get a rebate on those, plus you'll save more money in the short and long run.
4) The technology has not yet been perfected to make this cost effective. Better to wait a few years and see what kind of advances they make in the later models.
5) Sometimes hard to find qualified installers and dealers in your area. Coming out from the larger cities can cost extra.
The response number 1 is quite correct. However, some additions and clarifications to that answer will be useful.
Another major disadvantage of the solar water heater is that the maximum water temperature that can be achieved is significantly lower than what is possible with gas or electricity based water heaters.
Also water heaters can be installed only in locations that require good amount of sunlight. It is not possible install them indoors.
Also, I have different views on some of the points mentioned in above answer. I do not subscribe to the view that operation of solar water heater is very sophisticated.
Similarly, the point regarding portability is also not very convincing. The size of solar water heaters makes it more difficult to transport them, as compared to other types of water heaters installed in homes and other building. However, in general all types of water heaters are designed to be stationary rather than portable. When required, it is possible to design design portable heaters of all types.
1) it is very costly to install.
2) sun isnt there in the sky for 24 hours , so you cant heat during night time.
3) the operation of solar water heaters is very sophisticated.
4) the heater requires a large area to set up.
5) the heater is not portable.