(MagSafe power adapters have a chip built into it to prevent using an underpowered charger). If it's USB 2.x (USB Type A) or a regular type charger, it will either not charge or have the possibility of damaging the charger because the draw of the device will exceed the charger's ability.Since it technically doesn't have enough, it will either charge slowly or not at all.

If it's USB 3.x (USB Type C), it will negotiate power.

However, your 10W phone charger should not be used to charge your 87W MacBook. In other words, if your Apple USB-C charger is capable of delivering 87W of power, but you only need 10W for your phone, you can charge your phone with no problem. This applies to all chargers (USB, barrel connector, etc.), not just those that conform to the USB 3.x specification. This means that the device and charger will negotiate the power that is delivered.Īs to whether a charger is too powerful or not, you can always use a charger greater than or equal to the maximum power draw of the device. Part of that specification is the Power Delivery spec. USB-C conforms to the USB 3.1 specification. Yes, it can be used and you will not cause any damage. If there is only the MacBook 12 inch USB-C charger nearby, can it be used to charge other USB-C devices, such as any Android phones that also use USB-C? (will it be 'too powerful' and damage other devices?)

This answer is indented to be canonical answer for similar questions and applies to all devices that utilize USB (Type A or C) chargers and conform to the USB Power Delivery specification (version 2.x through 3.x).

Macbook usb c adapter