A solar water heater makes most sense if you live in a place where the weather is 50 degrees farenheit or higher year round. You can take advantage of the heat from our sun to raise the temperature of your water where needed. Here is a map to give you an idea of ground water temperatures.

The simplest answer for that is "when it accomplishes the results you are looking for at a price that is worth it to you". There are many ways to take advantage of the heat from the sun and direct it towards your needs. What I will be addressing here is the use inside your home. Lets say for example you live in Florida. Temperatures average between 60 and 70 degrees farenheit coming out of the ground depending on the time of year. If you have a large family the amount of hot water used for baths/showers, laundry and washing dishes would be much higher than with smaller families.

If you are looking at investing in solar electric as well, then it would make sense to utilize solar panels for water in combination with on demand water heating units. At around 120 degrees farenheit water becomes hot to the touch. So this would be the maximum temeprature you will be going for in most cases.

During the warmer months with longer days of sun, water temps from your solar water heating panels can reach into the 140 - 180 degree farenheit range. This makes for plenty of hot water with no other assistance from another source.

In the cooler months with water coming out of the ground at around 60 degrees farenheit and the heat from the sun raising the water temperature 30 - 40 degrees your water from your solar water heating panel(s) reaches temps between 90 and 100 degrees farenheit. A small hot water on demand unit raising the temperature 20 degrees will get you to the 110-120 mark. Saving you from having to pay for a larger on demand water heater unit that uses more electricty.

You have now accomplished an efficient means of utilizing renewable resources AND lowered your electrical needs without sacrificing your lifestyle.

Another example of when it may makes sense is where you have a radiator system in your home to keep you warm in the colder months. If you can preheat the water coming out of the ground that goes into your solar water heating unit before going to the your final heating of the water (electric, propane, heating oil, heat pump etc.) you have reduced your energy costs.

People are getting very creative with the combining solar with existing systems to save costs and the evironment. If you have an idea and would like specific answers or information on its feasability or how to make it work click here: Ask Solar Bob .