Why does my Macbook overheat and use so much CPU and energy when on YouTube?
You can test the efficiency of video encoding and decoding quite well by getting a video from Apple’s events page and playing it in QuickTime player.

Same in Safari - and then same in Chrome.

You’ll quickly find out which encoding are easy to run for the CPU and which have GPU acceleration.

When that’s done, you can see if Chrome or the app in question can choose a lower resolution or if you can download the file for offline playing so that it doesn’t hit the CPU so hard. Also, you might use an iPad or other far more efficient video handling device if you just want to watch videos with the most economy / least power.