Hi auto537, there is a total of 7 gas cans spread through the map, some more hidden than others. But you can always save on power by shutting off the cameras or the lights when not needed.
You can toggle them off from the same panel that is used to turn them on the second quest. Just flip down the switch labeled CCTV at any moment after completing the second quest. You can also toggle them back on at any point after that.
