You can make your own solar water heater. I am not responsible for how you use this information. This is for educational purposes only.
I made my own solar water heater using:
The immersion type heating element (with its accompanying sealing washer) and 1" NPT nut was the only thing I had to buy. Only the bucket was permanently altered for this project
I have 100 watt solar panels. At max power, they are 17.64 volts and 5.67 amps (17.64 volts x 5.67 amps = ~100 watts). To find the optimum resistance for the heating element, I take the voltage and divide it by the current. 17.64 volts / 5.67 amps = 3.1 ohms. So I need a heating element of 3.1 ohms. If my heating element has too high of a resistance, then the voltage will max to quickly for the panels and won't have as much current. If the resistance is too low, the panel will max on current at lower voltage. Both result in lower power transfered to the resistor.
Next, since I need 3.1 ohms, I must find a heating element of 3.1 ohms. Typically heating elements are rated by power and voltage. That just requires a little work on my part. To calculate a heating elements resistance you square the voltage rating (volts x volts) and divide this by the power. So, for example, I found a heating element of 600 watts at 48 volts. This is a safe element to use since my 100W panel cannot over heat the element and since the 17.64 volts will not max out the 48 volts of the element. Let's calculate the 600W and 48 volt heating elements resistance: 48 volts x 48 volts = 2304 volts squared. Now divided this by power. 2304 volts squared divided by 600 watts = 3.84 ohms. That's pretty close. The best heating element I have found to use would be 700 watts at 48 volts which gives 3.29 ohms. But it's too expensive for me.
I could not find a cheap 700 Watt 48 volt heating element. Hopefully someone who understands electronics and business will start selling this version.
I did find a 600 Watt, 48 volt heating element on Ebay for $15 including shipping. I bought this one. It has a resistance of ~3.8 ohms.
OK, with a heating element of 3.84 ohms, and peak power voltage of 18.6 volts, then I can expect to drive 17.64 volts / 3.8 ohms = 4.64 amps. Effective power into the heating element at maximum sunlight will be 17.64 volts x 4.64 amps = 81.9 watts. That's not bad. That's a ~82% efficiency in power transfer.
I chose to switch the negative side of solar panel. You can switch the positive side. It doesn't matter to the heating element.
I connected some big wire from the solar panel to the heating element. I didn't pay attention to polarity, because it doesn't matter with a heating element. Testing this, I can warm 1 gallon of water in about 1.5 hour.
While it is more efficient to heat water within a glass enclosure in a black bucket outside, this doesn't work well when the temperature is freezing outside. Also, you must carry the water back and forth. With a solar panel outside, and wires, you don't have anything that will freeze. You can just flip a switch (if you install one) to turn on the heating element. Of course you would want to make sure that you have water in the bucket before turning on the element.
This is a cheap setup, but interesting to know how it works, and how to do the calculations. A person needs to understand how it works to use it. You wouldn't want to heat the element without water in the bucket. This could melt the bucket or destroy the element and maybe start a fire? I haven't tried it to find out.
I currently have four 100 watt solar panels connected as 2 in parallel which is in series with another 2 in parallel driving this same heating element and can heat 5 gallons of water to shower temperature in about 2.5 hours. I get 37.3 volts and about 11.3 amps out of it max. I measured 330 watts of electrical power going into the heating element which calculates to be about 82% efficient.
I decided to go with a single pole, double through switch so that I can connect my "24 V" solar panel array to either the heating element or to my solar charge control, since my solar charge controller is a common anode setup. When I install this, I can simply switch the solar panel to either make shower water or charge the batteries as needed.
God has created an amazing heaven and earth. It's fascinating to learn and know the laws of God and use them in a profitable way to see his goodness towards mankind.
I hope you found this information educational.