I want to protect the inverter but do not know what size protection to use. It says peak power 900W. If I divide that by 12 I get 75A . I know the inverter has 3-35a fuses in it but do not know if those are redundant and back each other up so the fuse max is 35a?
Typically those 35A fuses will be in parallel - either directly, or as fuses to three switching circuits in parallel. Either way, the effective fusing capacity is just over 100A.
If you want to add an external fuse, it needs to be a propper dc type, of 120A or thereabouts. Adding an external fuse can add significant voltage drop if you are not carefull - thus limiting the inverter peak power.
