Many people believe that solar thermal water heaters are the best way to heat water with solar energy. This was true when solar panels were expensive per watt and when inverters lacked power. Back then, solar electric water heating made no sense. Now solar panels are cheaper and inverters are more powerful. This makes solar electric water heating a viable option that is well worth considering. So, we will look at what it takes to get a solar electric water heater vs a solar thermal water heater. We will then look at some advantages of a solar electric water heater.
Here is what you might need for a solar electric water heater.
Adding solar water heating means switching to an electric water heater. It also means building or upgrading your solar power system to accommodate it.
It is normal for an electric water heater to operate for about 3 hours per day while using about 4,400 watts. This equates to about 13.2 kilowatt hours per day. Divide this by the average sun-hours per day to get the additional solar panel wattage you need. That is, unless your solar array is oversized, which means you can add the difference. Now, keeping the system oversized for the new load will let it charge batteries faster even on a cloudy day. This will reduce the need for extra batteries.
If your inverter isn’t oversized, you will need at least 4,400 watts of extra inverter power. Otherwise, you can add the difference. Whatever the case, one inverter can meet this need. You will need a 240 volt outlet connected through at least a 20 amp circuit breaker. The outlet should be close to the water heater.
If you have backup batteries, they should be able to discharge at the rate necessary to power the increased load. Also, an increased load will need more batteries for the same duration of stored energy. An extra 13.2 kilowatt hours is more than enough, especially if you oversize the solar array. This step might be unnecessary if you already have plenty of battery capacity.
Here is what you need for solar thermal water heating.
You will need solar thermal collectors and plumbing that goes to them. You will need a hot water collecting tank and a backup water heater for long nights. The backup water heater can be a conventional model or a tankless water heater.
If you face sub-freezing temperatures, you will need an anti-freeze based system. The anti-freeze (usually glycol) will flow from the solar collectors to the heat exchangers in the hot water collecting tank and vice versa. This separate plumbing will need its own pump and expansion tank. This pump must automatically operate only in the daytime. Otherwise, it would speed up heat loss in the hot water tank at night.
With all this in mind, here are some advantages of a solar electric water heater:
There are fewer changes in plumbing.
If you use a solar thermal water heater, you will need plumbing that goes to and from the collectors. You will probably need pumps to send liquid to the collectors. Also, your plumbing will be even more complicated if you use an anti-freeze based system.
If you convert to solar electric water heating, you will only need to change your water heater to electric. Of course, that is, if you don’t already have an electric water heater. If you do, then no changes to this are necessary.
You can use it any time of day.
With a solar thermal water heater, your excess energy is stored in the hot water. You will slowly lose this energy during the night as the water cools. In the hours before dawn, you might not have enough hot water if you need it.
If you use a solar electric water heater, your excess energy can be stored in batteries or sent back to the grid. You will have access to hot water as long as you have electricity. This means that you can use it any time of day.
You won’t need a secondary water heater.
The water heated by a solar thermal water heater rarely stays warm throughout the night. So, if you need hot water later in the night, you will need a second hot water source. It is why solar thermal systems have secondary water heaters.
With solar electric water heating, you store energy in batteries. These batteries can store enough energy to last for days. So, you won’t need a secondary water heater.
When you don’t need hot water, you can use the energy for something else.
With solar thermal water heating, the collected energy can only be used as heat (usually for the water).
Adding solar electric water heating means doing 2 main things:
1. Increasing the energy handling capability of your solar power system.
2. Using some of that increased capacity to heat water.
This increased capability is useful for anything that uses your house’s electricity. This means the extra energy is useful for something else when heating water is unnecessary. You can even sell the extra back to the grid if you want to. This gives you a great deal of flexibility.
It is lower maintenance.
Adding a solar thermal water heater means adding a bit of extra plumbing to your home. You will need pipes going to and from the solar collectors, and you might need pumps too. Pumps and pipes can leak, and pumps can break. If you experience cold winters, you will either need to worry about the water freezing or about changing the anti-freeze. All these factors mean doing more maintenance.
Adding solar electric water heating will mean switching to an electric water heater. But there are no additions to the plumbing. Also, upgrading your solar power system doesn’t add any moving parts. So there should be no increase in maintenance.
In summary, solar electric water heating might have higher up-front costs. But this depends on your situation. Regardless, it has some advantages over solar thermal water heating. The first is that you will need little or no changes to your plumbing. You can also use it any time of day, including late at night. This means that you won’t need a secondary water heater. Also, the stored energy is useful for other things when you don’t need it for hot water. Furthermore, the simpler plumbing and absence of moving parts mean lower maintenance. With these advantages, a solar electric water heater is well worth considering.