Watt-Hours to Amp-Hours is a fairly easy conversion. Power in Watts is calculated by multiplying Amps by Volts. I always remember this basic conversion system by calling it PIE. P - Power (watts) I = Amps (that is the actual designation) and E = Electromotive force (aka Volts).

___P____

I x E

You can simply re-arrange the equation as needed to find the value or I = P/E.

Since we know Watts in Watt-Hours all we need to know is Volts. According to Shimano the mid-drive that bike uses is 36 Volts. So we just take 418Wh and divide it by 36 and we get 11.61Ah.