A solar panel can be connected to the external DC power input (VDC pin). In the photo below, a PowerFeather Protowing is used to break the VDC pin out into a barrel connector, to which the solar panel is connected.

Solar panel and battery connected to ESP32-S3 PowerFeather

The MPP voltage can be set from 4.6 V to 16.8 V to extract power from a solar panel more efficiently. Start with the figure mentioned in the solar panel, then experiment a little bit - set it a little higher or lower than this value.

The photos below illustrate the effect of not setting MPP voltage near panel rated MPP voltage, vs. setting it:

Battery current, supply voltage & current with MPP voltage not set

Battery current, supply voltage & current with MPP voltage set

Setting the MPP voltage leads to a significant increase in power extracted from the solar panel and delivered to the battery, illustrated in the table below.