how do I turn off PC using remote desktop with simple user account?
There is a need to restart PC that users often leave on.
PC has no local admin account.
Simple batch "shutdown" just writes that user doesn't have rights. Could it be possible with some services or programmatically in C# or using remote admin from inside app on the PC (I have access to domain admin) ?
shutdown.exe, but performed by an user that has the SeShutdownPrivilege priviledge. If the user has no privileges to initiate a shutdown with shutdown.exe, then it won't have the privilege with any other API. If such a way would exists, it would be a security vulnerability and that way would be called an 'exploit'.
Grant this privilege to your user so it can shutdown the PC. Open Local Security Settings MMC snap-in, navigate to User Rights Assignments, and grant this privilege, see Assign user rights for your local computer.