Thousand thanks; now it works perfectly:
For all others searching a StecaGrid solution:
// Steca PV - Inverter integration
String PV_AC_Voltage "AC_Voltage [%s] " { http="<[http://192.168.178.205/measurements.xml:6000:XPATH(number(//Measurements/Measurement[@Type='AC_Voltage']/@Value))]" }
String PV_AC_Power "AC_Power [%s] " { http="<[http://192.168.178.205/measurements.xml:6000:XPATH(number(//Measurements/Measurement[@Type='AC_Power']/@Value))]" }
and it works!