Using MQTT in Open Energy Monitor to Capture External Device Data

I struggled to find clear and well-organised information about this using web searches, so here is a condensed “how to”. The scenario I have is using a home-brew ESP8266 based device attached to my solar PV inverter which I want to relay definitive power output to my Open Energy Monitor via MQTT. This seems quite simple in principle, and is simple in practice, but seemingly not well documented.

First thing is to publish the data to an MQTT broker on the emonpi with a topic which starts “emon/{source}/{key}”, replacing {source} with a suitable name for the data source and {key} with the attribute name for the data being sent. In my case, I used “emon/solis/power” as the data source is the output power for my Solis PV inverter. The message payload is simply the data value. This immediately makes the published data appear on the “Inputs” screen of EmonCMS.

Two refinements/possibilities:

Send JSON

Rather than just a single value, send several in the same message. There are two ways to do this:

a) use a topic of form “emon/{source}”

b) use a topic of form “emon/{source}/{key}”

If the message payload sent to the MQTT topic “emon/solis/power” looks like {“ac”: 90, “dc”: 120}, option (a) creates EmonCMS inputs “ac” and “dc” under “solis”, while option (b) creates inputs called “power_ac” and “power_dc”.

Include a timestamp

To do this, simply include an extra field in the JSON called “time”, with a value which is the Unix time. If you are testing, the Unix time needs to be fairly close to the actual time (ignoring summer time) otherwise EmonCMS will indicate “inactive”, but it still captures the data.

Aside: I used the VSMqtt plugin for VSCode as the MQTT client, as I’m using the PlatformIO plugin to develop my ESP8266 code (using the Arduino libraries). Update: these days I’ve changed to using MQTT Explorer as my go-to software for viewing MQTT messages.