An IoT Kits customer in Singapore asked to measure the energy collected by the solar panel. A simple modification of a D1M-WX1 weather station kit made this an easy project.
Details of the project can be found in this GitHub repository:
The modification to the D1M-WX1 kit is facilitated by the fortunate coincidence that the INA219 voltage/current sensor module has the same pinout as the BME280 environmental sensor used in the kit. It is an easy matter to replace the BME280 with the INA219 and upload new firmware to the D1 Mini.
The INA219 is a very accurate current and voltage sensor. It has the additional benefit of including a multiplier to calculate power as the product of voltage and current. The voltage, current, and power measurements of the solar panel output are sent to the D1 Mini over the same I2C bus used by the BH1750 light intensity sensor.