look for 16 awg 18 awg is dangerous imo
16 awg? I doubt you will find that. The wire in the wall on a 15amp circuit is 14 awg and that is rated to carry 1800 watts. 20amp circuit uses 12 wire. 18 gauge is definitely enough to carry 150 or so watts.
No, 14 AWG wire is rated to carry 15 AMPS at up to whatever voltage limit the insulation is good for - this has NOTHING to do with watts.
18 AWG at 12 VDC will NOT handle 150 watts comfortably, as that is 12.5 AMPS and 18 AWG is only rated for something like 7 or 8.
Quite a few high-quality power supplies will use 16 AWG wiring on their PCI-E dual-connector cables because it's NEEDED to handle the load from both connectors safely - some use it on all of their PCI-E cables just to save the trouble of keeping more than one size around and having to worry about mixing them up.