Why is my MacBook Pro battery draining so fast?

I've been working on an 80,000 page book in MS Word and using a browser for research.

I also use Word for books and articles, and must often have a browser open for my research. I've found that what you see is most likely due to your browser or the web pages you have open.

I can run Word alone for a long time on battery, even on a geriatric Macbook Pro I use for travel. It has an 11 year old battery. Once I open certain web pages to fact-check, I can hear the single fan spool up in reaction to internal heating and see the charge level icon start to drop. I close the web page and all is normal again..

Chrome is probably the most resource-greedy of all the browsers:

but I can see demand go up on a site-by-site basis even in Safari. If you cannot live without Chrome, it's time to plug in.

I just did a test. With only your question open in Safari, discussions.apple.com was using about 5% energy per Activity Monitor. Then I opened the OWC site (eshop.macsales.com) where I shop a lot. When I reached an individual OWC product page, look what happened:

Impressive, and my CPU temps rose by 10°C within minutes of opening that page

I ran into similar situations when looking at election night result pages. All in Safari.

So before worrying about your Mac's battery, do what has already been wisely recommended: let Activity Monitor reveal any guilty parties.

View energy consumption in Activity Monitor on Mac - Apple Support