[karmakaze]

Historically batteries have had known standard voltages (12V, 6V) and within that context, mAh is easily convertable to Wh.

With USB PD and powerbanks etc that operate at many voltages using mAh is unclear. It's only used historically as a number comparable to previously used numbers. As someone mentioned the implied voltage is that of the cell 3.7V

So the conversion formula is Wh = mAh/1000 * 3.7V (or substitute the standard voltage for the context)