This is a short article on fusing solar or other charging sources. It can also be helpful for other voltage sensitive equipment.
When fusing devices such as solar controllers, alternator output, wind or battery chargers they need to be fused close to the battery (source), or within a few inches of a change in wire gauge. It can be helpful, from a voltage drop perspective, to use the largest size fuse the wire can handle. Doing so will minimize any added voltage drop across the fuse.
I often hear "My solar panel can only put out 4.5A so I will use a 5A fuse.".
Unfortunately this is a flawed mind set to be in. You should ideally eb thinking; "My 10GA 105C rated wire has a max ampacity of 60A outside of an engine compartment so I should use a 50A - 60A fuse."..
This photo shows a 5A fuse at a 5A load. It is adding .1V of voltage drop to the losses already there in the wiring and terminations.