Charge macbook 2015 from ipad charger
The iPad charger will take approximately 3x longer to charge than using the standard charger, if your numbers are correct. This may not be fast enough to charge the laptop while you are using it, depending on how much energy it needs.

Try leaving the laptop plugged in while you aren't using it. You can also access the Activity Monitor and go to the Energy tab to get an estimate on how long it will take to charge from the AC.

It is also possible the USB A to C converter you have only supports data transfer and not charging.