On my 2017 iMac under Mojave 10.14.6, I use both the iWork suite of Pages/Numbers/Keynote and the Microsoft equivalents in the Office suite of Word/Excel/Powerpoint. I can’t speak to the battery life differences since the iMac is plugged in, but the work I have to do is so much greater in Office. The Office apps do many, many more things than the iWork ones, and those are things I just don’t need. I can save iWork docs in formats compatible to Office and that seems to work well. Worst case: use *.pdf. About the only thing I find I can do faster and more easily in Office than iWork is a mail-merge. Office still makes it hugely cumbersome, for me anyway, but I don’t think I’ve even gotten it to work in iWork, or if it even does it.
And when I say Office, I exclude OneNote, Outlook, Teams, whatever, just Word/Excel/Powerpoint. I do think the new unified version of these 3 is helpful.
One way to save battery is to either turn off (not a good idea) or relocate auto save. If you have to connect every few minutes to a server somewhere online, it will use more battery than storing locally and auto saving to your local device.