Use these water heaters as a dump load to use your excess power for water heating.
As an example you can set your charge controller to turn on the load at 14V and off at 13.5V and then it will turn on the water heater to use the excess power. Bear in mind these voltages should be tweaked for your specific system, charge controller, battery bank etc.
Victron Smart Battery Protect 65A and 100A 12V/24V can be used to directly switch a dump on/off at the voltages above.
If using simple heating elements you must source a thermostatic switch for them, which we do not supply or sell.
NOTE - Use of any heat dumps in DIY fashion should be overseen and approved by a qualified installer.
More relays, switches etc can be found here - click here