This is a common Arduino project for which it is very well suited. Since you intend to retrieve the values once a month or so, the obvious solution is to record the data to on-board SD card. The magic commands are
myfile.print(data);
and
myfile.print(",")
and thereby accumulated as a .csv file which may be read directly into Excel.
The file(s) may be eventually retrieved by extracting the card or simply downloaded to a phone via Bluetooth.
There are datalogging and filedumping examples included with the SD library. You will probably have an on-board RTC, and you can also use that to name files by date.