Using iMac 5k 27" as external PC monitor

Use your iMac as a display with Target Display Mode - Apple Support

"iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays."

As Malcolm J. Rayfield has already mentioned, you won't be able to use this iMac with Display Mode which is designed to be used with other Macs. Even if you could, the iMac would need to be powered-on to work, defeating your second goal.

xSOMx wrote:

I'm planning on switching to a PC, but I'd like to keep the 5k retina display and use it as a monitor for my PC. Is it possible to do this using a Thunderbolt-to-HDMI adapter? And does the actual iMac computer have to be switched on when used as a monitor, or is it possible to "decouple" the actual display from the computer and use it externally? I'm thinking in terms of my electrical bill, which ought to be twice as high if I have essentially two computers running at the same time, even though one is only used as a monitor.

Thanks.

Bottomline, there is no way to use your iMac as a direct video

display for any computer, Mac or PC as that feature was removed on

5K iMacs as previously pointed out.

However, depending on your use, you could install Microsoft Remote Desktop Mac

and view your PC over a network. Anything that would be graphically intensive

may not be suitable, but most everything else should be fine.

xSOMx wrote:

Thanks for the answers everyone!

.....And how come this feature was removed with the 5K iMacs? ...

Only the engineers and marketing people at Apple

know why it was removed.