Did you read the Manual?
https://docs.px4.io/main/en/getting_started/px4_basic_concepts.htmlIf you still have questions I'd suggest to contact them directly.
Battery Capacity requirement will depend on what the Pichawk draws and what all other devices connected also draw.
Capacity is typically in units of A-Hrs for batteries. W-Hr can be used but one must consider the dropping Voltage of the battery as it discharges.
You also need to consider the total timer the unit is powered on. This includes the Launch setup, flight and the recovery )this can be hours if flight is not nominal).
Seems you also need to study power requirements, basic electronic engineering. I say it this way since you are at University and there to learn.
Quick overview---
Current is DRAWN from a source (Battery) by the LOAD.
Voltage is Supplied by the Source.
I quick look at the Pichawk schematics (on their web site) indicates it requies 5V Regulated source. The Manual indicates the Pichawk's power comes through a power module.