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I'm looking to purchase a new MacBook Pro and a monitor, and I'm currently looking at the 15" MacBook Pro (2018) with the LG UltraFine 5K Display.

I'm fully aware that the MacBook Pro and the LG UltraFine 5K Display are compatible with each other, as indicated by this Apple Support page.

Having said that I noticed that the 15" MacBook Pro ships with an 87w power supply, whilst the LG UltraFine 5K Display provides up to 85W of charging power via the USB-C input.

Does that mean, that if the 15" MacBook Pro was performing some form of heavy computation, such as video rending, it could start to use the additional power requirements from the battery, and thus require the original charging cable to give it sufficient power (obviously at some point the battery will go flat)?

I'm probably looking into this far more than what is required, but it does seem strange that the latest 15" MacBook Pro laptops require more power than the "official Apple supported" monitor can provide.

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For MacBooks (including Pro), if the requested voltage based on current system load is higher than the power available, it will try to make up for the difference by drawing from the battery; if the requested voltage is lower than the available power, it will use the extra power to recharge the battery until it is full.

For using the 85W charger on an 87W MacBook Pro, the difference is so small that it is virtually unnoticeable. In rare cases, your battery might be losing a small amount of power slowly when the machine is under max load. In most case, it will be just fine.

Note: The LG UltraFine 5K is made for MacBook Pro 15" and MacBook Pro 13", and you can see that MacBook Pro 15" has the best feature support. So it is safe to assume that everything, including the power delivery over USB-C, is highly optimised.

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All MacBook products will request power and then apply the power they receive to match what's delivered.

In practice, you can run at a deficit for minutes or hours and still catch up on charge once the power draw is lower than the supplied.

Long term, over driving a 29W charger with a 87W load will shorten the service life of the fully run charger, but I've not ever seen this happen in practice. Also, in practice the 61 watt charger from Apple generally keeps up with any and all 15 inch MacBook Pro so much that people prefer to travel with a physically smaller adapter and don’t miss the 25 or so extra watts.

Also - I've not had the LG cinema displays skimp at all at powering an aggressively run MacBook Pro previous generation with far too many power hungry accessories connected like Thunderbolt 3 and Thunderbolt 3 to Thunderbolt 2 adapters powering multiple spinning hard drives, USB powered scanners and more.

The power supplies in these displays wouldn't make me think a moment to run them with the newest loaded computer based on my experience with them so far. Especially ao over a < 3% over power situation you quote from general specifications.

  • You cannot overdrive a charger (asking 29W charger for 87W); the charger will cap the battery at 29W and the device being charged will just have to live with (if possible) the lower charge. For MacBooks (including Pro), if the requested voltage based on current system load is higher than the power available, it will try to make up for the difference by drawing from the battery; if the requested voltage is lower than the available power, it will use the extra power to recharge a battery that is not full. – williamli Sep 11 '18 at 10:03
  • @williamli Very good points. What wording might be better than “over drive” - I was trying to speak that perhaps they are engineered to assume an average daily output of 10W on average for 8 hours of duty (delivering 29W for 2 hours and then 7 W for 6 hours) would add more thermal stress than the design life of 5 years at a lower duty cycle. Not that it would run at 45W for a short time type of “over drive” – bmike Sep 11 '18 at 18:57
  • Would you consider making an edit to clean up the wording @williamli - I’m not sure how to be succinct about thermal loading. – bmike Sep 11 '18 at 18:58

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