The stock USB-C power adapter for Macbook 12 (2015/2016) charges 2.0A at 14.5V (29W) (while it also supports output of 2.4A at 5.2V). The Macbook battery is 41Wh (or 39Wh for the older model) so the charging time is around 2 hours.

The Xiaomi 16000mAh powerbank charges Macbook around 1.4-1.8A so the charging speed is around 1/4 of the stock power adapter. The total amount of energy in the Xiaomi 16000mAh powerbank is 57.6Wh so it should be roughly enough (because of voltage conversion, losses etc) for one full charge. It should be able to charge Macbook from 0 to 100% overnight (8 hours). (Actual test to follow later)

The same goes for the smaller 5000mAh version of Xiaomi powerbank. It just charges slower and has significantly less capacity. Roughly to fill 1/3 of total Macbook battery capacity.

Just like powerbanks, even regular AC to USB-A adapters charge Macbook 12. The adapter in the following picture is rated 5V 2.4A so the actual output likely matches the maximum while the voltage is much more stable than from powerbanks. So a good USB-A charger will charge Macbook at 1/3 speed of the original power adapter.