diff --git a/tools/esp8266/app.cpp b/tools/esp8266/app.cpp
index d45baa6..682cf15 100644
--- a/tools/esp8266/app.cpp
+++ b/tools/esp8266/app.cpp
@@ -10,6 +10,9 @@
 #include "html/h/setup_html.h"
 #include "html/h/hoymiles_html.h"
 
+//SD_logger
+#include <SD.h>
+#include <TimeLib.h>
 
 //-----------------------------------------------------------------------------
 app::app() : Main() {
@@ -287,27 +290,38 @@ void app::loop(void) {
             mMqtt.sendMsg("uptime", val);
         }
 
-        if(mSerialValues) {
-            if(++mSerialTicker >= mSerialInterval) {
-                mSerialTicker = 0;
-                char topic[30], val[10];
-                for(uint8_t id = 0; id < mSys->getNumInverters(); id++) {
-                    Inverter<> *iv = mSys->getInverterByPos(id);
-                    if(NULL != iv) {
-                        if(iv->isAvailable(mTimestamp)) {
-                            for(uint8_t i = 0; i < iv->listLen; i++) {
-                                if(0.0f != iv->getValue(i)) {
-                                    snprintf(topic, 30, "%s/ch%d/%s", iv->name, iv->assign[i].ch, iv->getFieldName(i));
-                                    snprintf(val, 10, "%.3f %s", iv->getValue(i), iv->getUnit(i));
-                                    DPRINTLN(DBG_INFO, String(topic) + ": " + String(val));
-                                }
-                                yield();
-                            }
-                        }
-                    }
+    if (mSerialValues) {
+      if (++mSerialTicker >= mSerialInterval) {
+        String logFilename = (getDateTimeStr(mTimestamp)); //get current time in format yyyy-mm-dd hh:mm:ss
+        logFilename.remove(10, 9); //remove hh:mm:ss from yyyy-mm-dd hh:mm:ss including space between dd and hh
+        logFilename += ".csv";
+        //Serial.println(logFilename); //for debug only
+        File dataFile = SD.open(logFilename, FILE_WRITE);  //open or create file on SD card
+        mSerialTicker = 0;
+        char topic[30], val[10];
+        for (uint8_t id = 0; id < mSys->getNumInverters(); id++) {
+          Inverter<> *iv = mSys->getInverterByPos(id);
+          if (NULL != iv) {
+            if (iv->isAvailable(mTimestamp)) {
+              for (uint8_t i = 0; i < iv->listLen; i++) {
+                if (0.0f != iv->getValue(i)) {
+                  snprintf(topic, 30, "%s/ch%d/%s", iv->name, iv->assign[i].ch, iv->getFieldName(i));
+                  dataFile.print(topic); //write to SD card
+                  dataFile.print(","); //write "," to SD card in same line for csv format
+                  snprintf(val, 10, "%.3f %s", iv->getValue(i), iv->getUnit(i));
+                  dataFile.print(val); //write to SD card
+                  dataFile.print(","); //write "," to SD card in same line for csv format
+                  DPRINTLN(DBG_INFO, String(topic) + ": " + String(val));
                 }
+                yield();
+              }
             }
+          }
         }
+        dataFile.println(" "); //start new line in logfile
+        dataFile.close();  //close file on SD card
+      }
+    }
 
         if(++mSendTicker >= mSendInterval) {
             mSendTicker = 0;
diff --git a/tools/esp8266/esp8266.ino b/tools/esp8266/esp8266.ino
index 4ddd706..822de03 100644
--- a/tools/esp8266/esp8266.ino
+++ b/tools/esp8266/esp8266.ino
@@ -14,24 +14,39 @@
 #include "app.h"
 #include "config.h"
 
+//SD_logger
+#include <SPI.h>
+#include <SD.h>
+const int chipSelect = 16; //SD-CS pin GPIO16 = D0
+
 app myApp;
 
 //-----------------------------------------------------------------------------
 void setup() {
-    myApp.setup(WIFI_TRY_CONNECT_TIME);
+  myApp.setup(WIFI_TRY_CONNECT_TIME);
+
+  // TODO: move to HmRadio
+  attachInterrupt(digitalPinToInterrupt(myApp.getIrqPin()), handleIntr, FALLING);
+
+  if (!SD.begin(chipSelect)) {
+    Serial.println("SD card failed, or not present");
+    // don't do anything more:
+    return;
+  }
+  Serial.println("SD card initialized.");
 
-    // TODO: move to HmRadio
-    attachInterrupt(digitalPinToInterrupt(myApp.getIrqPin()), handleIntr, FALLING);
 }
 
 
 //-----------------------------------------------------------------------------
 void loop() {
-    myApp.loop();
+  myApp.loop();
+
+  
 }
 
 
 //-----------------------------------------------------------------------------
 ICACHE_RAM_ATTR void handleIntr(void) {
-    myApp.handleIntr();
+  myApp.handleIntr();
 }
diff --git a/tools/esp8266/main.cpp b/tools/esp8266/main.cpp
index ce96cbc..193cf29 100644
--- a/tools/esp8266/main.cpp
+++ b/tools/esp8266/main.cpp
@@ -311,6 +311,7 @@ void Main::showUptime(void) {
     snprintf(time, 20, "%d Tage, %02d:%02d:%02d", upTimeDy, upTimeHr, upTimeMn, upTimeSc);
 
     mWeb->send(200, "text/plain", String(time));
+
 }
 
 
@@ -447,6 +448,7 @@ String Main::getDateTimeStr(time_t t) {
     else
         sprintf(str, "%04d-%02d-%02d %02d:%02d:%02d", year(t), month(t), day(t), hour(t), minute(t), second(t));
     return String(str);
+    
 }