If you’re wondering what’s best, a solar water heater or a tankless water heater, the answer may be BOTH – or NEITHER! If you have an existing gas water heater that is in good repair, you may just want to add a solar water heater with additional storage capacity. If you have an electric storage water heater, you may want to retrofit your existing tank or add additional storage with a solar tank.
There is a great article about the choice between a storage and tankless water heater on the Green Building Advisor website here. You should check out this article to learn some of the pros and cons of tankless water heaters. When the question of solar comes up, the decision becomes a bit more complicated. How much storage do you need? Do you need an endless supply of hot water? How old is your existing heater? What are your usage patterns?
The beauty of a solar water heater is that you can preheat water for a primary heating source. By providing an electric or gas element with water that is preheated, you will drastically reduce or even eliminate the energy needed to reach the final desired temperature. In Southwest Florida, our incoming water temperature is around 77°F (Source: EPA). To heat a pound of water one degree, it takes 1 BTU of energy. One gallon is 8.33 pounds. So, to heat a typical 50 gallon storage tank of water to 120°F, a typical desired set point, it requires 17,910 BTU of energy. That’s about $0.60 if you are an FPL or LCEC customer in SW Florida.
If you use a solar water heater to preheat water and store it in a secondary storage tank, you significantly reduce the amount of energy required to heat water with the primary heating element. If you have modest hot water usage, the water coming into the primary tank will always be preheated to some degree. It takes the same amount of energy to heat water from 77°F – 87°F as it does 110°F to 120°F. Any heating that you do with solar energy reduces your heating costs proportionally. You will only pay a fraction of the $0.60 required to heat a 50 gallon tank if the incoming water is warmer than 77°F!
When used with a tankless instant-on gas heater, solar water heaters often provide a water temperature that is higher than the desired set point. To deal with this, a tempering valve is installed for safety to avoid scalding. It also requires that the gas element is “full modulating,” meaning that the burner can go down to zero heating when the incoming water exceeds the desired temperature. In this case, there is NO cost to heat water – the solar water heater does all of the work! This significantly changes the economics of a tankless water heater, reducing maintenance and gas costs and extending the life of the heater.
In some cases, a tankless heater makes the most sense. In others, a traditional electric tank heater is the best choice. Adding solar water heating will reduce heating costs, and to what extent depends largely on your usage quantity and patterns. In most cases it makes sense to integrate solar with another heating source, and in some circumstances, solar water heating is not advisable. If you are a very light hot water user or only spend a limited amount of time at your Southwest Florida vacation home, solar water heating may not be the most economical choice. Larger households and people who eat and do laundry at home are usually heavy hot water users and benefit tremendously from solar water heating.
Since the decision depends largely on your particular circumstance, it’s best to get a free consultation from a solar professional. A reputable solar dealer will ask you questions and find out how you use hot water and advise you on what solar can do for you!