wiring an inverter

[Brite Idea]

I'm installing a 1500 watt inverter with a 20 foot run of #2awg wire from the batteries to the inverter. The question I have is "do I run it off the back of the disconnect switch or direct to the batteries"? I read the manual and it says to install a 150amp fuse right after the power source. I know that will protect it under load but when it's not in use is the on/off switch enough or should it have its own disconnect switch?