Well you can start here by attempting to read the file. ESP32 Arduino SPIFFS: Reading a file - techtutorialsx
It does it one character at a time. You have to assemble each line in a buffer and parse it.
It looks like you are writing lines in this format into the file at the moment:
snprintf(dataString, sizeof(dataString), "\n%3u, %6.3f, %3u", currDay, dTotalkWh, mWatts);
I guess your temporary file would need only dTotalkWh