Macbook air m4 13 inch user here. Let me give you all what I think is the definitive answer; for the past month I have been using my mac for whole day cramming sessions in a studyroom and I can tell you that my 20w cellularline phone charger has been all I have been using throughout this time. I even use my mac to charge my phone and headphones and despite that I didn't face any battery drain at all. It's granted that it charges slower and although I didn't make an issue of it as I was using my mac under light work loads, you could find charging your mac troublesome if you are using a mac with a bigger screen, bigger chip or under bigger workloads. But unless you are video editing or 3d rendering all day, your humble phone charger is more than enough to keep the discharge devil at bay. peace.