When working out what size fuse to use with your inverter a good way to work this out is as the example below.....
For a 3000 watt 12 volt inverter:
3000 divided by 10 = 300. So the size fuse you would require for a 12 volt 300 watt inverter is a 300 amp Mega Fuse.
For a 3000 watt Inverter 24 volt:
3000 divide by 20 = 150. The size fuse you would require for a 24 volt 3000 watt inverter is a 150 amp Mega Fuse.
The most common used fuse to use would be a Mega Fuse.
The fuse should be sized from the surge rating of the inverter e.g. Sinergex Pure Sine 3000 watt 12 volt has a surge of 6000 watts that would mean for a 6000 watt inverter 12 volt: 6000 divide by 10 = 600. So the size fuse you would require for a 12 volt 3000 watt inverter is a 600 amp Mega Fuse.