Solar water heaters are very old and proven technology, with roots in ancient times. Modern solar water heating collectors can capture about 85% of the available solar energy in a given area. That’s far above the ~20% harnessed by commercially available solar electric panels. A question often asked is whether someone should buy a solar water heater or a solar electric system, but the answer is not always obvious.
One of the solar water heaters we sell has a rating of 12.5kWh/day. This rating is based on a particular climate and rating category (relative water temperatures). What this rating tells us is that the collector is capable of transferring the equivalent of 12.5kWh of stored energy to water on an average day. That’s like heating about 100 gallons of water by 50ºF (for example, 75-125ºF) each day. To offset the same amount of electricity each day, you would need a solar electric array rated at about 2.4kW, which would cost substantially more — almost twice as much.
So the answer seems obvious — before you invest in a solar electric system, you should buy a solar water heater, right? Not so fast…
The first question to ask is whether you really use that much hot water. Smaller households rarely use that much hot water. Larger households, especially those with children, use a lot of hot water, and are much more likely to benefit from solar water heating. The next thing to consider is whether you will heat water every day. Some days households use lots of hot water for laundry and dish washing. Other days may not see much water use at all. In fact, while on vacation or out of town seasonally, a solar water heating does effectively nothing. There are really no savings to consider.
That 12.5kWh rating becomes effectively much, much less if you don’t actually need to heat water! If your needs were for about half that amount of water heating, you would essentially be indifferent between a solar water heating and solar electric panel investment (because the initial cost of an equivalent solar water heater is about half). If you are a very light hot water user, the analysis will favor solar electric panels.
What makes it even harder to decide is that your existing water heater is not metered – you probably have no clue how much electricity it uses. Your overall household electricity use, on the other hand, is metered by the electric company. It’s easy to know how much energy you use. Perhaps even more important – once a solar water heater is installed, there is no cost effective means to measure how much it actually saves you, and how you can maximize your benefit.
While it may sound like I don’t want you to consider a solar water heater, the opposite is true. If you feel you are a heavy hot water user, you should go this route first. In fact, I just installed a solar water heater at my home, knowing that I do not economize on my hot water use, despite having just a two-person household. Having copious amounts of hot water, heated economically, is important to me.
There are two other factors to consider, one in favor for solar water heating, and one against. In favor, you can have heated water in times where electricity is not available (think post-storm hot showers!) Against, solar water heaters typically have 10 year collectors warranties, which is less than half that of 25 year warranties on solar electric systems.
While this analysis makes a decision clear as mud, the point is that you should strongly consider your water usage and other factors important to solar water heating before deciding what technology is right for you. An experienced solar advisor should be able to help you make a decision, which is often arrived upon using a combination of objective and subjective measures. Don’t rule out a small solar electric system as an alternative to solar water heating, and consider a solar water heater first if you are a heavy hot water user.