Most mains energy meters have an LED which flashes 500 or 1000 pulses for every measured kWh. A photo-diode mounted in front of this LED detects the flashes. Every pulse is converted to a clean digital signal by an adjustable comparator with hysteresis followed by a 74HC14 Schmitt-trigger. The resulting pulses can be inverted by means of a jumper. A falling edge triggers an interrupt of the ATMega328. The firmware counts the interrupts and calculates the equivalent energy in Wh. Every 60 seconds the result is published to the MQTT broker on topic 'distributioncabinet/energy/solar'.