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 a minimum of 1 hour of full sunlight, we are going to select the solar panel.
From the previous step, it is concluded that the average current consumption is 7.5 mA
Charge required for running the device for the whole day = 7.5 mA x 24 Hours = 180 mAh
So, our target is to generate 180 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 = 180 mA at a voltage of around 5 to 6 volts.
Solar panel rating = 180 mA x 5V = 0.9 Watt, by considering some losses, I have selected a higher rating solar panel.
Solar Panel Selected: I have used a 5V,250mA Solar Panel ( 110 x 69 mm)