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 or 24V 600W Screw In 1 INCH BSPSolar Water Heater Element Immersion Water Heater

Unit comes configured for 12V.  There are 2 elements which are connected in parallel.  To make it 24V change the configuration into series by removing the serial connections and putting a connection between the top terminals to make it wired in series.

Specification:
1. Material:SUS304
2. Diameter: 8mm
3. Total Length: 220mm
4. Volt:12v or 24v
5. Flange:1"BSP