The fuse between the battery bank and the controller is an ABYC wiring standard requirement. Everything between the battery bank and an appliance must be fused to meet the standard and pass boat inspection.
To determine the size of the fuse needed for the solar system, do the following calculation: Watts of solar /12V (or 24V) = amps + 10 amps. For example: 240W solar / 12V = 20 amps +10 amps = 30 amp fuse. Also, many controllers have a built in fuse as a secondary protection. The protection is against a short circuit overheating the wires to cause a fire. There is no reason for a fuse between the solar panels and the controller although many customers use a circuit breaker there for the switch. To calculate the capacity of the switch: Watts of solar / Voc (open circuit voltage) = amps For example: 240W solar / 30V = 8 amps. Note the much lower amperage on the solar side of the controller vs the battery side of the controller in the examples above. That's MPPT. Panels never surge power unless hit by lightning.
Thomas Trimmer has been cruising with his Ericson 38 sailboat on the Great Lakes for over 20 years. He has pioneered the use of solar energy for wilderness cruising. He is continually designing and building equipment to simplify and enhance the cruising experience.