Watt = voltage x amperage
A given motor at a given load will always have a wattage rating, whether you know the number or not.
You want to find the amount you would need to run it under the conditions you wish to attain.
Amperage output capacity is your C rating times your battery capacity rating in amp hours.
Eg: 25C, 2200mah battery = 25 x 2.2 = 55amps capable
Given 7.4v or 11.1v with a 25c & 2200mah rating, both will output 55 amps. However the voltage will change how much watts get to the motor, to wit, how much wattage the motor will be able to use.
7.4v x 55amp = 407watts
11.1v x 55amps = 610.5watts
There is also the wiring to take into consideration, but the voltage drop in all but the worse of wiring should be less than .5v, so its negligible.
