I know that I can make my Macbook Pro hibernate so that I can take the battery out and unplug power. It works just like Windows hibernate, but better because it's a Mac. I don't know if it will still work on new Macs or new OS X, but I see no reason why not.
See here:
http://apple.stackexchange.com/questions/126669/how-to-add-hibernate-mode-to-macbook-pro
Here's some info specific to iMacs:
https://discussions.apple.com/thread/4931350?start=0&tstart=0
And specific to El Cap:
http://www.idelta.info/archives/some-hidden-changes-in-os-x-el-capitan/
By default sleep on desktop Macs requires power to maintain RAM. You can enable Safe Sleep which keeps everything in RAM for fast resuming, but saves a copy to the disk just in case. I don't see any reason not to do this, except there will always be a file on the disk equal to the size of RAM. You might not want to do it on a SSD, but I've had it enabled on my SSD for years. Once it's done saving safe sleep data, you can unplug the power.
I don't know how this would work with Power Nap. That periodically turns on the computer in a low power state without the display or fans on to run maintenance tasks. If you unplug it during that, you would probably lose data. I assume if you unplug it right after it hibernates then power nap won't run.
I use a dock widget called Deep Sleep that makes my mac hibernate immediately when I click on it. I do this when I travel because otherwise normal sleep mode slowly drains the battery. Power management wasn't that good in 2008, you know.
https://www.apple.com/downloads/dashboard/status/deepsleep.html
I have no idea if this still works with El Cap.
Sorry if any of this is outdated. I'm still using a Macbook Pro 4,1 with 10.6.8.