MacBook Pro 2018 battery drains when connect to an external monitor

Hello, I have the newest 2018 MBP 15" with Touch Bar. 2.2GHz, 16GB RAM, Intel UHD Graphics 630 1536MB.

Whenever I connect the MBP to an external monitor (I happen to have the Dell S2718D), the battery drains very quickly (like, one percent every 1-2 minutes or so), and the computer physically becomes pretty hot (in the upper right hand quadrant of the computer). I'm connected to the monitor via an Apple USB-C to HDMI converter, and the monitor has its own power source so I don't understand why the MacBook is working so hard, especially when I'm only browsing the internet and using Word/Excel.

I've read about 2017 MBP having a similar issue. Something about a stronger, more "battery-sucking" graphics card that kicks in once connected to an external monitor. Perhaps this is the issue. When my MBP is NOT connected to an external monitor the "about this computer" shows in the details/description only one graphics card: the Intel UHD Graphics 630 1536MB. However, once connected to an external monitor, both the Intel AND a Radeon Pro 555x 4096 MB graphics card are listed in the "about this Mac".

All that to say:

1) Is there any other issue that may be causing the battery drain/heat that you can think of? Is there some setting to turn on/off? Is the monitor somehow the issue?

2) Is the graphics card(s) the problem? Is there some sort of setting to only use the one, less-powerful graphics card?