When you start a night, you are allowed a limited amount of power, starting from 99%. Once that power runs out, you must spend the rest of the night without your camera, and the doors to your office will be opened, leaving you at mercy to the animatronics. Doing anything in the game will result in loss of power, looking at the camera, keeping the doors closed, and turning on your lights will result in losing power at a faster rate. Power can also drain when Foxy's run is unsuccessful.
Power tends to run out faster the closer you are to the end of the week, and animatronics are faster, so use your power wisely.
At the bottom of the screen near your percentage shows the amount of power you are using up. The more bars you have, the faster power will run out. The usage bar goes up if you're using more devices (The lights, doors and camera) all at the same time. Power usually goes down every 1 minute on the first bar, and usually goes down about every 5 seconds on the last bar.