explanation about BLDC motor current
So you know the power of the machine, 20kW. This will more than likely be shaft power.

\$P = T\omega\$

Power equals torque x speed.

You should have such information with regards to what this machine is to be connected to. What is important for battery current capability is the Torque.

\$T = k_t i\$

Torque = torque constant x current.

The beauty of BLDC machines is this current is the DC value and it only flows in two phases, thus is the current that will be drawn from your DClink (which may be your battery)

Make sure your battery voltage is of suitable voltage to achieve your operating speed with regards to all voltage drops (switches, cable, inductor)

This peak current will flow from the battery so initially you must ensure the battery can satisfy your peak torque point.

So to amp-hours. You need to quantify your duty. Do you operate at maximum speed and torque but only for 1second, the rest of the time a low speed low torque point.