I also have a 300W inverter, only ever used a couple of times to charge my MacBook. If I were to do it over, I would ditch the inverter and get one of these:
https://www.amazon.com/Trianium-Charger-Chipset-ENBL-Macbook-Chromebook/dp/B0711BK418/ref=sr_1_2?s=electronics&ie=UTF8&qid=1519226924&sr=1-2&keywords=usb-c+car+charger+macbook
It is pretty silly to take 12V DC, convert it to 120V AC, then convert it back to 20V DC for charging. Now that cheaper DC chargers are available and devices are standardizing on USB/USB-C charging an inverter makes less sense.
https://www.amazon.com/Trianium-Charger-Chipset-ENBL-Macbook-Chromebook/dp/B0711BK418/ref=sr_1_2?s=electronics&ie=UTF8&qid=1519226924&sr=1-2&keywords=usb-c+car+charger+macbook
It is pretty silly to take 12V DC, convert it to 120V AC, then convert it back to 20V DC for charging. Now that cheaper DC chargers are available and devices are standardizing on USB/USB-C charging an inverter makes less sense.