From the previous step, it is concluded that the average current consumption is 9.3 mA

Required Current for running the device for the whole day = 9.3mA x 24 Hours = 223.2 mAh

There is no current gain in the linear regulator used in the WeMos, so any current used at 3.3V results in the same current at 3.7V or whatever voltage the battery is at.

The amount of solar insolation varies according to which part of the globe you are located at. To find out the amount of solar insolation in your area, you can use the Global Solar Atlas. By taking consideration into minimum 1 hour of full sunlight, we are going to select the solar panel.

So, our target is to generate 223.2 mAh in 1 hour.

To charge a 3.7V Li-Ion battery, a solar panel of voltage 5 to 6V is adequate.

Required Solar Panel rating = 223.2 mA at a voltage of around 5 to 6 volts.

Solar panel rating = 223.2mA x 5V = 1.1W

Solar Panel Selected : 1W / 5V to 6V

In this project, I have used a 5V,200mA Solar Panel ( 99 x 69 mm)

So a 1W panel should be enough the run the project even in winter in places with a high latitude.

Note:If your location receiving ample amount of sunlight, then a 0.66W solar panel which I have used in my earlier version also work.