Here are some solar panel basic thoughts and terms.
Voc solar panel open circuit voltage. The highest voltage the system will see and what you need to consider with charge controller, CC, input; breaker and fuse ratings.
Isc solar panel short circuit current; use this for wire size, fuse size or breaker size between solar panel and CC.
Vmp, typical operating voltage of the panel under load. Used to determine panel wattage rating.
Imp, typical operating current of the panel. Varies with battery state of charge, sun intensity and shading of the panel. Used to determine panel wattage rating.
Panel watts = Vmp * Imp or wattage the panel can produce in lab conditions. Varies greatly in real world conditions.
Vmp and Imp are maximum power point values and are what the MPPT, or Maximum Power Point Tracking controllers try to follow to get the most wattage to the battery.
About fuses or breakers, The NEC recommends de-rating by ~20% but nobody to my knowledge can find where this applies to RV's but it can't hurt to err on the safe side. Voc is used when choosing voltage ratings of fuses or breakers. Every solar panel I have seen to date has a maximum fuse or breaker size marked on the panel.
Voltage used when determining charge rates or panel size is typically 14.5 volts for flooded batteries. Typical range is 14.2 to 14.5 volts and is referred to as the absorb voltage. This needs to watched very closely on Gel batteries as they don't like really high voltages.
When a question was posted about running the reefer on dc and panel size I used the absorb voltage in the calculation I posted.
When looking at the discharge side or wattage used from the battery the typical voltage used is 12 volts.
Now that we are on the battery side of things always use a breaker or fuse between the CC and the battery(s). I prefer to use breakers over fuses for two reasons. One is to reduce failures, fuses will fail over time more often then breakers. Two, breakers give you a disconnect means.
One more thought on breakers they must have an AIC, this is an arc in air extinguishing value, of at least 5KV or 5000 volts and need to be marked as such. Most house hold breakers have a 10Kv rating but this isn't a DC rating.
You can use some Square D breakers but be very careful here about Voc exceeding the 48 Volt breaker rating. Yes, there are three ratings for breakers; current, voltage and AIC. For the DIY I can't stress caution here with breakers enough. Same thing applies to fuses and fuse holders.
When dealing with batteries and short circuits a very nasty and intense energy release occurs resulting in a very high fire danger.
Add in the possibility of hydrogen gas being present and look out.
My follow up post to this will be ways to determine solar panel sizing.
@Bill feel free to edit any mistakes of mine.
@rickst29 If you spot anything wrong here send me a PM.