How do I diagnose battery drain whilst asleep?
I have not found anything that satifactorily answers my original question of how to see historical battery usage (Apple calls it "Energy Impact" in Activity Monitor).

But to diagnose what might be happening over the time period I followed the process here: 2016 Touch Bar MBP drains battery in sleep of looking at system logs.

There were a lot of repeated errors from GoogleSoftwareUpdateAgent throughout the day.  I don't know if this is the cause of battery drain, but in case it is, I followed these instructions to remove it: https://superuser.com/questions/730736/how-to-remove-google-software-update-from-mac-osx

On High Sierra, it was:

$ ~/Library/Google/GoogleSoftwareUpdate/GoogleSoftwareUpdate.bundle/Contents/Resources/GoogleSoftwareUpdateAgent.app/Contents/Resources/ksinstall --nuke
$ defaults write com.google.Keystone.Agent checkInterval 0