Measuring solar power with an Arduino
Just a few points:

You can't measure DC current with a coil; you would need a Hall-effect sensor;
The charging current and the voltage you measure will depend on the state-of-charge of the battery, making it difficult to compare measurements;
You can't fully charge a nominally 12V battery with a 12V solar panel; a 12V lead-acid battery with the panel you mention in the comments should work;
You need at least a diode between solar panels and battery, and possibly a proper charger, depending on the chemistry and capacity of the battery and the maximum charging current; maximum charging current for a 12V 9Ah lead-acid battery is about 1A if you stay on the very safe side, so the panel you mention should be OK; just don't overcharge the battery;
You need to be sure the battery voltage doesn't exceed 12V by too much, or the voltage divider will put more than 5V on the Arduino's input pin and fry it. Nominally 12V lead-acid batteries are usually charged to about 14V, and the solar panel you mention can get there. This would put 5.6V on the Arduino input pin with the resistor values you use in your voltage divider.

Also have a look here: Measure power of solar cell with an esp8266.