When you cut the power and the guard is still alive, he will come to turn the power back on. Run past him as he comes through the doorway. He will say " Is someone there?". Run through the room. When you come back, you will find out that the lights are off and the guard is gone. He is around the corner. Take out your rifle and kill him. With most guards, when they say "Is someone there?", they stand perfectly still and you can shoot them easily.