You say you installed Manjaro, that probably explains your battery woes. Pop!_OS does alot good battery optimisation, I generally wouldn't recommend getting a System76 laptop if you don't like it out of the box.
Thanks for the suggestion. PopOS was not yet created when I purchased mine, it came with Ubuntu 16.04LTS. I used Ubuntu up until a couple months ago and decided to make the switch instead of upgrading to 18.04. Honestly I think my battery life has improved slightly but I haven't performed any benchmarking and my evaluation may be biased.