watts = amps * volts
Use that to figure out what size fuse to use. Make sure your using the surge watts (max) for the inverter and not the normal watts. Surge is usually double the normal. So if you have a 500 watt inverter, it's probably 1000 watt surge. That alone is 83 amps. Of course thats at full power. But you dont want to design the system half ***. Youve got the power so design for it, it will make upgrades in the future much easier. Speaker watts are different than power watts. If your amp puts out 50 watts to 4 channels, that doesnt mean your using 200 watts. Its actually lower than that. Im not sure how much but to be safe you could just use the 200 watt figure, just know that its not what youll actually be using. The wire will depend on how far your running it. Here is a table to help you figure the wire gauge. You'll need at least 4 gauge. Use 2 gauge if you plan on going more than 8 feet. Dont run 2 wires. Its sloppy. Use one good wire and run it into a fused distribution block. That way you will have a main fuse at the batterey and individual fuses for each device at the other end. This is the dist. block that I used and it works good.