diff --git a/README.md b/README.md
index e0005e7..02a29c4 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,10 @@ Features:
 * needs 5V power supply
 * opensensemap.org API
 
+Power consumption:
+* ~50mA @ 5V
+* (45mA idle + fan + wifi)
+
 CAD files (OpenSCAD) and source code included.
 
 
@@ -72,6 +76,20 @@ RadiationD-v1.1(CAJOE):
  - 5V  -> 5V
  - VIN -> D6
 
+
+12V power mod with step down converter and voltage divider for voltage measurement:
+
+  BAT+ --+--------------.
+         \R\            |
+          \1\           '
+            >---+--------------------------------- A0
+          /R/   |       .    ________________
+         /2/   === C1   +---| buck converter |---- 5V
+  BAT- --'------+-----------|________________|---- GND
+
+  Example values: R1=1MΩ, R2=100kΩ, C1=some ceramic capacitor (optional)
+  measures up to ~30V
+
 ## ToDo
 * https://hackaday.com/2014/10/25/use-a-cheap-pin-diode-as-a-geiger-counter/ (opengeiger.de/elektronik.html)
 * Solar power
diff --git a/src/main.cpp b/src/main.cpp
index c5457be..0007d37 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -27,8 +27,6 @@ extern "C" {
 #include <SDS011.h>
 
 const char* server = "ingress.opensensemap.org";
-//const char* ssid = "Freifunk";
-//const char* password = "";
 
 #define MQTT_MAX_PACKET_SIZE 512
 const char* mqttserver = "home.xd0.de";
@@ -38,6 +36,8 @@ const char* mqttpassword = "password4";
 constexpr unsigned int postingInterval = 60000; //Uploadintervall in Millisekunden
 constexpr unsigned int dhcp_interval = 60*60*1000;
 
+#define EXTERNAL_POWER 1
+
 //senseBox ID
 #define SENSEBOX_ID "5a9e9e38f55bff001a494877"
 
@@ -73,7 +73,9 @@ static constexpr uint8_t GEIGER_PIN = D6;
 static constexpr float CONV_FACTOR = 0.008120;
 static constexpr float OWN_BACKGROUND_CPS = 0;  // documentation says 0.2 (make sure value doesn't get negative if subtracting!)
 
+#ifndef EXTERNAL_POWER
 ADC_MODE(ADC_VCC);
+#endif
 
 ESP8266WiFiMulti wifiMulti;
 
@@ -147,7 +149,11 @@ void ICACHE_FLASH_ATTR sendValues() {
   sd.pressure /= 100;
   bmp.getTemperature(&(sd.temp2));
 
+#ifndef EXTERNAL_POWER
   sd.voltage = ESP.getVcc()/1024.0;
+#else
+  sd.voltage = analogRead(A0)*0.04285078 -0.05942125; // by linear regression for my(!) voltage divider. else: sd.voltage = analogRead(A0)*3.3*(R1+R2)/(R2*1024)
+#endif
 
   int sds_error = sds.read(&(sd.p25), &(sd.p10));
   if(!sds_error) {
@@ -170,6 +176,7 @@ void ICACHE_FLASH_ATTR sendValues() {
   if (!sds_error) Serial.printf("Particles 10 : %6.2fµg/m³ (SDS011)\n", sd.p10);
   if (!sds_error) Serial.printf("Particles 2.5: %6.2fµg/m³ (SDS011)\n", sd.p25);
   if (sd.cpm > 0)    Serial.printf("Radiation    : %6.2fµSv/h (J305)\n", sd.radioactivity);
+                  Serial.printf("Voltage      : %6.2fV     (ESP8266)\n", sd.voltage);
 
   DynamicJsonBuffer jsonBuffer;
   JsonArray& array = jsonBuffer.createArray();