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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.

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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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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