Köszi, így jó lett:

String dataString = "";
dataString += String(LAKASFUTES);
dataString += ",";
dataString += String(fusthofok);
dataString += ",";
dataString += String(T1);
dataString += ",";
dataString += String(T2);
dataString += ",";
dataString += String(T3);
dataString += ",";
dataString += String(T4);
dataString += ",";
dataString += String(T5);
dataString += ",";
dataString += String(T6);
dataString += ",";
dataString += String(T7);
dataString += ",";
dataString += String(kW);
dataString += ",";
dataString += String(szazalek);
dataString += ",";

File dataFile = SD.open("datalog.txt", FILE_WRITE);

// if the file is available, write to it:
if (dataFile) {
dataFile.println(dataString);
dataFile.close();
}