The default Laser Turret is 800kj per shot, and it shoots once every 40 ticks (1.5 shots/second). 800 * 1.5 = 1.2MW, which is the default (non-bonused) energy consumption of the turret.
The GPL fires every 6 ticks (10 shots/second) but each shot also does more damage (46 vs 20). 46/20 = 2.3 * 800kj = 1.840 MW per shot * 10 shots/second = 18.4 MW energy consumption. 1MW = 1MJ.
Edit: I need to read. You were talking about the personal laser turret, which is more efficient than a normal laser turret, for some reason. My bad.