What size fuse to use?

ditto that. 80 no matter how many watts, huh?...................

First off fuses do not rate by "watts"..they rate by amperes.(amps) and voltage (in this case we are discussing 12v of course)

Secondly, the MAIN fuse you will use near the battery is to protect the wire. If you use a fuse that is too small it will blow. If you use a fuse that is "too large" this in itself will NOT "cause" any damage per say. In other words the system is only going to draw as much current as it needs to operate..no more..no less. At times a "surge" may cause it to draw more current for a short period of time which is why going over the rating is generally reccomended, within reason.

Here is what the fuse will do. If there is a short (a power wire touches ground) or lets say a component goes bad causing excessive current draw, the fuse will blow. This will prevent wires from burning if it is shorted. However if the excesive draw is due to a faulty piece of equipment the fuse will not save it. The fuse did not cause the damage regardless but it could help in a sense that further damage to the component will be minimized due to circuits burning up. However, the equipment still needs to be serviced, using a larger fuse will not fix the problem.

So it's really simple, no need to read too much into it. With amplifiers generally they will have a fuse or fuses included. So lets say for example the amp has 2-30a fuses. That means the amp is not designed to draw more then 60a under ANY circumstance. So using an 80a fuse would be perfectly safe. If they made a 70a fuse this would be ok too but generally they run 30, 40, 50, 60,80, 100...etc. now you could probably use a 60a but as stated earlier a "surge" could cause it to blow and using a slightly larger fuse would prevent this without any problems under normal use. Another option is a breaker. This is more expensive BUT it serves the same purpose as a fuse. The advantage is it can be reset. Many of them have a reset switch as well so you can kill the main power to the system easily during service. Kind of nice.

I see a few replies discussing wire gauge. IMO this is only for making sure you use wire large enough to meet the current demands of the system. However you still want to use a fuse slightly larger then what the amp can pull at max power, and that rating is simply the fuse(s) that come with the amp. No rocket science needed at all. If the amp for some strange unbeknownst reason has no fuse or current rating you can use the ohms law with the rated max power output to determine current, and use a fuse slightly larger.