iflyit is in fact correct. Watts consumed is a function of the current being drawn out of the battery at the given voltage of the pack. Ohms law is pretty basic, E=I x R. E is the voltage of the pack, I is the current drawn from the pack due to the load & R is the load on the pack or the servo. R is a constant in this case, the servo is the servo is the servo. A 4 cell pack puts out a nominal 4.8 volts and Ohms law says Servo X at voltage (4.8v) is going to draw Z amount of current. When the voltage increases & the servo stays the same, the current pulled from the pack is going to increase as well. So a 4.8 volt 2700 mah pack is going to deplete slower then a 6 volt 2700 mah pack.

Sad but true

edited to get the facts straight.