This simple-ish device takes a voltage reading at Arduino pin A0, and logs it
to EEPROM with a timestamp from a DS3231 RTC module. When you press the
button, all stored data will be written to UART at 9600 baud and the EEPROM
will be reset.
The green LED (D1 on pin D9) blinks when it takes a reading, and the red
LED (D2 on pin D10) becomes lit when the EEPROM is full. 1K of EEPROM can
hold 146 7-byte records, that's 36.5 hours at the default sample rate of 15
minutes.
Resistors R3, R4, and R5, form the current shunt. It is tapped 1/3 of the
way in to make sure the analog input is not over 5V for the small 5V/1W panel I
was testing. You will probably need to adjust the current shunt for a bigger
panel. It's pretty close to the limit for a 5V/1W panel.
The DS3231 module I used is a tiny cheap one designed for a Raspberry Pi.
Really useful, and often less than $1 USD on eBay.