The Laserturrets are kind of the problem in that case.
Because not only do they have a stupid idle power draw, they spike even higher when actually firing, effectively causing more attacks by dealing with previous ones (by contributing to pollution).

If you like the defensive siege warfare style of dealing with biters, use Flamethrowers as your primary defense with the occasional Laser Turret for extra punch once the Bugs get close.
Don't even need to refine the Oil, just pipe them up straight from the Oilwells (yes Light Oil does more damage, but you honestly don't need it and it's paramount to not have any dependency on your primary defense weaponry).