My Dell laptop doesn’t have any hot key to turn the monitor off. I was using the software which turns off monitor when I lock the system but unlocking is always an extra step there. Push Off Monitor is small executable which solves this problem.
After you start the program it sits on computer’s tray icon and if you press SHIFT + F1 it just turns the monitor display off. To resume back just make mouse or press key movements and the it turns it on. Download Turn off monitor from here