.. so use Safari and Sublime.

People seriously need to start ditching apps built on Electron and start using native applications again. Atom uses about 400-600Mb memory. So does GitKraken, and so does WhatsApp. The first two can be replaced by Sublime (rarely above 200Mb and much faster) and Fork, although the only real alternative for WhatsApp is Telegram (with the native client), although that has not nearly as much users.

I don't use Chrome or even keep apps like Slack open anymore.

If I'm just browsing and chatting, battery life is excellent.

But if I want to use Pro apps (XCode) on my Pro laptop, battery life takes a nosedive. I understand that, but in the 2016 revision they reduced the battery capacity, and it sounds like they based that decision on "well, Safari and Mail are super-efficient, when you're doing "Wireless Web" you can now get the same battery life as with the old one, why would anyone need more capacity?"

I switched to mostly Safari when I got the new MacBook Pro because I understood that there were efficiencies built in that would maximise battery life. Not sure how large a difference if any there really is. In any case I get a few hours when all I'm doing is light browsing. No Atom open, etc. Which is for me OK as I'm usually not away from a socket for very long.

The figure of 15.x hours, however, seems completely unbelievable to me.