I'm sure there is a formula, but since the motor is an AC motor and is therefore dependent on frequency, inductance, and load, it would be quite complex. Much easier to measure the current and voltage drawn from the batteries and do your calculation there. Even then, the power drawn will change dramatically based on the acceleration. A device like the eagletree systems unit takes voltage and current samples every so often (100ms IIRC) and then you can upload that to a PC to get a graph of the power usage.