We've got a range of laptops in our office all macbook pros some of which are 13" and some of which are 15".

The 15" laptops came with 85w chargers, and the 13" came with 60w chargers.

I want to buy some additiaonl chargers to leave plugged in around the office for anyone to use, but im unsure as to which type i should get.

  • Would charging a 13" laptop on a 85w power supply damage it ?

  • Would charging a 13" laptop on a 60w power supply damage it, or just make it slow at charging ?


No. You cannot damage a laptop by charging it with a charger rated more than what it originally came with.

When a device has a power requirement of 10W it means it will only draw up to 10W. A power supply with a rating of 45W will supply up to 45W.1

A general rule of thumb is that you can always get a bigger power supply, never smaller. Think of it like your electrical circuits in your house. In the US, it's common to have 15A or 20A circuits. Can you plug in vacuum cleaner that uses 12A? Yup. Can you run 2 at a time (24A) on the same circuit? Nope, it will pop. Can you run 1 vacuum and a 1A lightbulb (13A)? Sure.

So, you can plug a device with a 60W draw into an 80W power supply, but it's not recommended to plug the same device with a 60W draw into a 45W power supply.

What can happen?

With the new USB-C ports on the Mac, most likely a slow charge. With devices in general - you could damage/destroy the device and/or the power supply.

1 This assumes the same voltage. USB-C will auto negotiate the power delivery. Older devices that don't do this; too many volts can fry your device. More amps than necessary won't harm anything. Also see: Choosing power supply, how to get the voltage and current ratings?

  • Thanks @allan so i shouldn't plug my 15" macbook pro (it came with an 85w charger so i presume thats what it will draw) into a 13" macbook pro's 60w charger. Does magsafe 1 auto negociate what is required ?
    – sam
    Feb 5 '17 at 17:23
  • You "can" plug it in, but I don't recommend it. Magsafe includes a chip in the power connector that identifies itself to the power connector. What may happen is a computer that powers, but doesn't charge (or vice versa). Once someone borrowed my Magsafe adapter and replaced it with a lesser power one and I spent a week trying to figure out why my MB wouldn't charge and would only power up with a full charge.
    – Allan
    Feb 5 '17 at 17:42
  • I'd say this is half nitpick, half useful correction: Too many amps will definitely fry your device. Volts alone are harmless, but too high a voltage will induce a current that will fry things. So if a 60W device has an 85W charger, (provided proper current control is in place), it will just pull less amps and run at 60W. So really, too many volts available may fry your device, too many amps available might not.
    – Rafael
    Jun 28 '20 at 14:25
  • @Allan, strange, I never knew that could happen. I've been charging a 15" macbook pro with a 60W charger for months and nothing bad happened.
    – Rafael
    Jun 28 '20 at 14:28

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