The size of this fuse is dependent on how many solar panels you have and how they are connected series parallel or series parallel.
Fuse between solar panel and controller.
With an 8 awg copper conductor between the controller and battery you can use a fuse or breaker up to 80 amps using a 90 degree conductor insulation in free air.
That does not mean you should use a 80 amp ocpd it just means that is the largest you can use in your application.
The first version of this system used 20a class cc fuses and a bulky fuse box meant for home solar.
I need to put a fuse between the battery and the solar controller.
The second fuse between your solar panels and charge controller is a little bit different to figure out.
The load terminal of the solar controller can be connected with a dc power device which is the same working voltage as the rated voltage of the battery and the controller supplies power to the load with the battery voltage connect the positive and negative poles of the load to the load terminals of the controller there may be voltage on the load side please be careful to avoid a short circuit when connecting the load a fuse should be connected to the positive or negative cord of the load.
For that setup you will not need fuses between the solar panels and the charge controller.
These are off grid so we ll have 55w solar panels charging 12v 100ah deep cycle batteries during the day and the sensor gnomes use that battery power to listen for bats at night.