Adobe Support's position: "Photoshop is a huge program. That's why it takes more CPU. It is normal for any application to use CPU while it is running on the computer."
FYI here's the response I got from Adobe, edited just for brevity:
You are now chatting with Mrinmay Majhi.
Mrinmay Majhi: I would like to inform you that there are lots of processes running in the background for Photoshop.
Mrinmay Majhi: Like Save in Background option in Photoshop
canton becker: There are no documents open
canton becker: With no documents open, photoshop is consuming more CPU over the course of 24 hours than my operating system, email, browsers, finder, etc. *combined*.
canton becker: It's only about 14%, but it's 14% non-stop.
Mrinmay Majhi: As I said, whenever you run any application on your computer, it will use some CPU percentage, as there are many processes that is running to keep the application ready for you.
Mrinmay Majhi: Photoshop is a huge program. That's why it takes more CPU.
canton becker: Okay, I want to make sure I have this right. The official response is that it is perfectly *normal* for Photoshop CC 2014.1 to consume 14% CPU when the application is completely idle without any images open. Because it is a "huge program". Correct?
Mrinmay Majhi: Canton, I would suggest you to close Photoshop if you are not using it.
Mrinmay Majhi: Whenever you want to use it, you can open it and start using it.
Mrinmay Majhi: If you keep any application opened on your computer, it will take more or less CPU.
Mrinmay Majhi: It is normal for any application to use CPU while it is running on the computer.
One ray of light: in many cases, when I quit photoshop and open it again, the CPU goes back to 0. I think it's only when photoshop is idle *after* I've done some work, and then closed all the windows.
Zero percent here even with 3 images open (but not working on them obviously)
For a while after switching out of Photoshop, the app will write some of the scratch data to disk.
Some Flash and HTML panels can consume time while in the background (due to bugs).
And we've seen a few GPU drivers consume time when they shouldn't.
But the most common cause of background time is font previews (and frequently corrupt fonts). Try disabling font previews and see what happens.