I have a few USB devices that are powered by USB. When my MacBook is on battery I assume the draw from USB device is from the only power source, the battery of the MacBook. When the MacBook is plugged into the outlet, does the USB device draw power from the battery or the power outlet?
2 Answers
Neither.
Technically, it get’s it’s power from the 5V power rail and if using USB3.1 Power Delivery, it can pull from the 20V power rail as well.
Where do those rails get their power? From the main rail (normally PPBUS_G3 HOT). This is what feeds the MacBook from either the power adapter or the battery. It’s the SMC that manages power distribution throughout your MacBook.
In short, you could be getting power from battery only, from the adapter only, or a combination of the two (usually the case).
It will draw from whatever the MacBook's current power source is. If the MacBook is plugged in, the USB devices will draw power from the wall outlet and not the system battery.
An exception in some models could be if the MacBook is running a heavy compute load. In this case it might max out the power adapter and draw from battery power to get extra power as well.
