The monitor comes with a USB-C port that sends power as well as image. But it's unclear to me:
- What's the wattage it sends?
- Is that enough to charge an MBP that's in use?
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Your model MBP ships with an Apple 87W USB-C Power Adapter and USB-C Charge Cable. However, you don't need the full 87W in order to be able to charge the MBP. Apple's adapter is designed to provide an ideal amount of power in order to both use the MBP and charge it at a certain rate.
You can use lower rated power adapters, but depending on the quality and actual power output, they may not last long if used continuously for charging the MBP, or they may not be able to charge it while it's operational. Obviously, I can't cover all scenarios here.
However, in terms of the U2419HC Dell UltraSharp 24 USB-C Monitor, one of the USB downstream ports does provide up to 65W of power. By way of comparison, Apple typically provides:
In a nutshell, what this means is that you will be able to use the U2419HC Dell Monitor and also charge your MBP at the same time, but it won't charge your MBP as fast as your Apple 87W USB-C Power Adapter will.
How fast it will charge your MBP will also depend on its usage - if you're just doing some word processing it'll charge faster than it will if you're rendering videos or playing graphics intensive games, so that may factor into your considerations. However, for most people most of the time, this monitor will work fine as a display and charging source.
Since you're interested in understanding the rate of charge and discharge of your MacBook Pro, I thought I'd recommend you download and try coconutBattery. It provides a whole heap of information that you'll find useful, and from memory includes both Discharging with and Charging with measurements in watts (obviously, only one is displayed at a time since your device can't do both simultaneously).
Also, I should point out that macOS also provides a great deal of information. Follow these steps:
Got to Apple > About This Mac
Click on the System Report... button
In the left pane under the Hardware heading, select Power (it's about 2/3 of the way down)
On the right-hand side, scroll down to view the AC Charger Information
Note: I am not affiliated with coconutBattery in any way.