The basic formula to calculate current draw (amperage) is:
W / V = A
1200 watt invertor.
12 v system.
1200 / 12 = 100 Amps Current.
You should take voltage drop into consideration. If the vehicle voltage were down to 11.5 while you have a device drawing a large amount of current, your draw would increase to 104 amps. When the current draw increases, the voltage will naturally drop. It's all part of that silly stuff they call physics.
I would recommend getting something like a Stinger 150 circuit breaker. The advantage two a circuit breaker is its ease of reuse. A power invertor is a typically a very dirty power source (ever heard that whining sound), and they often have very unregulated power supplies. That will vary its current draw slightly.
Also, the circuit breaker needs to be very close to the battery - i always try for less than 12" - but you need a decent mounting surface, so you just do the best you can given the engine compartment. God forbid there were a short on the hot side of the breaker, you could almost bet a paycheck that you're about to see a fire. The amount of current a 2ga line can withstand on a short run to a battery is extremely high, and that energy is going somewhere - heat.
They also offer a marine version if you wheel/mud a lot. Is like $20 more or so. Probably worth it.
You can most def use a fuse if you don't want to spend for a circuit breaker. On stingers website, you can find the part numbers to aching you want to purchase - then just eBay it. I don't think stinger sells to the public.
Hope it helps...