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This question pertains to USB-C not MagSafe. MagSafe negotiates available power with the Dallas 1-wire protocol. USB Power Delivery (PD) is an unrelated protocol. Answers based on experience with MagSafe are unlikely to be relevant.

I'm considering buying a 2016 MacBook Pro Retina about which this article says:

the Griffin product was only powerful enough to offer 60W of power, not quite enough for the 87W needed to power the larger Pro models at fully

I'm also considering buying a LG 27UD88-W which it advertised as providing:

Mobile / Laptop Charging Up to 60W

What are the consequences of using a 60W supply instead of an 87W one?

Does it just mean it will take longer to charge the laptop?

Could it have side effects such as putting parts of the system into a slower, low-energy mode?

This question covers MagSafe power supplies but Charles Duffy's comment suggests that the rules for USB-C may be different.

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    This is a quite different question between USB-C and non-USB-C-based products. It looks to me like the preexisting answers aren't appropriate to the specified hardware. – Charles Duffy Jan 5 '17 at 16:47
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    @CharlesDuffy — I've updated the question to make it more explicitly about USB-C if you'd like to nominate it for reopening. – Quentin Jan 5 '17 at 16:53
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    Unfortunately, I don't have the relevant permissions here -- I'm only a high-rep user over on StackOverflow -- but hopefully someone monitoring the review queue will pick it up. – Charles Duffy Jan 5 '17 at 17:00
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    I bought the LG 27UD88 and was wondering about the same question. In particular, I wondered if USB-C could be any different. In the event using a 60W power is harmful to my MBP, there is nothing I can do to disable power delivery from the 27UD88. It turns out Apple has the official word on it: it won't damage your MBP. I wrote more about my research here: unsolicitedopinion.net/2017/07/11/… – junjie Jul 11 '17 at 1:14
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Apple partnered with LG to create a 4K monitor that outputs 60W via USB-C. Apple has mentioned full compatibility with a 13" MacBook Pro (comes with 61W power adapter) but mentioned that with a 15" MacBook Pro, the battery will get drawn during intensive power usage and therefore it should be connected to its 87W power adapter. No mention of any dangers in doing this.

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I am an Apple Certified Mac Technician working for an Apple Authorized Service Provider.

I and my co-workers have seen this occur multiple times. When the MacBook tries to pull more power than the power adapter can supply, it will do one (or more) of three things:

  1. Burn out the power adapter internally
  2. Cause physical burn marks (scorching) on the MagSafe connector
  3. Damage the internal power control circuitry on the logic board

The first two issues are easy enough to fix; you can replace the power adapter and the MagSafe board inside the computer for a (relatively) low amount of money. If the internal circuitry is damaged, you'll be in for a new logic board which can easily be $500 + labour, and Apple will not cover this as it will be considered "accidental damage".

The MacBook Pro 2016s have not been out for very long, and our shop hasn't yet seen the damage that can be caused by under-powering a USB-C charging port. That being said, I expect that much the same symptoms/issues will occur in that case.

As answers on the MagSafe thread state, just get a full-wattage power adapter. Consider the slightly-higher cost as your insurance against needing to replace the logic board in you brand-new 2016 MacBook Pro.

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    USB-C PD (Power Delivery) allows (rather, requires!) a negotiation between the charger and the device. I'd be very surprised if you could get these symptoms when using a MBP with a compliant charger and cable, unless Apple botched their implementation of the standard. – Charles Duffy Jan 5 '17 at 16:48
  • @CharlesDuffy I bow to your superior google-fu. As a tech who does not enjoy telling customers that they've burned out their logic board, I hope that you are correct :) – Joel C Jan 6 '17 at 18:16
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    I use my (Nexus 6P) USB-C cell phone charger to charge my MacBook Air on a fairly regular basis, so I'm literally willing to put my money where my mouth is here. If Apple got their implementation right for the MBA, I can't see them goofing it up for the MBP. – Charles Duffy Jan 6 '17 at 18:21
  • Damn, now I understand why my last MBP was getting heated at the magsafe connector. It was IT staff who mixed matched chargers and MBPro and I thought it is okay. – Thamme Gowda Jul 8 '17 at 6:41
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    Outcome 1 certainly is possible (but if this happens with an original Apple charger I'd seriously have to question Apples engineering prowess). However 2. and 3. make no sense whatsoever from a physics perspective. How can a lower current cause burns on a MagSafe port that is rated for a higher powered charger? The same argument applies to the charging circuitry. – UloPe Jul 28 '17 at 10:43
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According to this article from Apple,

The best way to charge your MacBook Pro with Thunderbolt 3 is to use the AC adapter that came with the computer. Using an underpowered AC adapter, like those that come with iPad, will make charging slower.

From that, I deduce it's an accepted practice and that there's no harm in doing that.

What it's not so clear to me is whether an underpowered laptop runs in power adapter mode, battery mode or what, and how that affects its performance.

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