Use these water heaters as a dump load to use your excess power for water heating. You can set your charge controller to turn on the load at 14V and off at 13.8V (or 13.6V) and then it will turn on the water heater to use the excess power. If you are using high powered dumps you will need to use a relay to switch the load on.
Stainless Steel DC 12V 300W Screw In 1 INCH BSPSolar Water Heater Element Immersion Water Heater
2. Diameter: 8mm
3. Total Length: 220mm