[brian015]

You are using power (Watts) which is Volts x Amps. The power used over a period of time is the energy (Watt-hours, or W·h). You want to know the energy a particular pack will provide, so multiply its Volts x Amp-hours to get Watt-hours. A 3s 5000mAh pack is 11.1V and 5.0Amp-hours, so 55.5 W·h. A 6s pack (double the voltage) would need to have a capacity of 2500mAh (half the capacity) to give you the same amount of energy.