The voltage is variable during the ride so will not give an accurate calculation ?Maybe a stupid question, but why do you need this value? If you like to know the value you can easily calculate it by deviding the power (W) by the voltage (V). The voltage is roughly 36V so the outcome should present the Amperage (A).
P=UxI
Power (W)
U = Voltage (V)
I = Amperage (A)