Which servo you use will tell you how much power you're going to need. Some servos work on 4.8 volts only, some on 6.0 volts only and others will work on both. Digital servos generally require more amperage than analog as well. For one of my boats, I use the Futaba S9156 digital servo for the rudder. This thing works fine on a standard Futaba 4.8 volt nicad pack but will operate faster and have more torque using a 6 volt pack, as stated on the package and under the tech notes on the Tower Hobbies web site