SHELLY EM Contactor Power Control with Dual Power Metering

I recently took posession of a Shelly EM device and a pair of Shelly EM accessories – i.e. split core current transformers. The EM product is straight-forward enough – 2 pairs of polarised inputs from the transformers, live and neutral input – and an output (which I’ve not yet used). The EM has overconsumption protection on the output, intelligent on-off via 2.4Ghz WiFi, optional SSL connectivity and embedded web server (nothing new there, then).

Shelly EM and transformer

SO, the POINT of this was to optionally control my air-source heating but importantly to monitor whole-house power and heating separately as the heating costs are significant. I’m not sure of the significance of the connectors on the transformers (one shown above – I have two) but I cut them off as the EM has no matching connectors – just the usual screw fixings. I hooked up the transformers and mains power to the EM. I fastened the EM to the back of my cupboard wall which has our house breaker box in it and wrapped one transformer around the incoming LIVE lead., the other around the LIVE power to the heating system (the cool thing about non-contact is I didn’t have to switch anything off during all of this).

I then powered up the Shelly APP on my Android phone, told it about the nearest WiFi access point at which point the APP noticed the EM had turned on it’s own access point. I told the APP to connect EM to MY access point and in seconds, that was it. I declined the offer of using the cloud (as I have a VPN for remote access). And that’s the pretty part done- now to see my cupboard (which is pretty ugly and overcrowded).

Shellly EM MQTT output to Grafana

I checked the reported overall power consumption – which agreed with our Smart Meter (though the Shelly has slightly less lag). I’m not 100% convinced about the APP (which often takes ages to show a connected device on opening the APP but thankfully, The EM like other Shelly devices can handle local MQTT without flashing 3rd party software. The green glitches in the graph are down to me messing with an immersion heater – the bulk of power is the horrendous consumpion of air-source heating. The rest of the house by comparison uses very little (unless my wife has her washing machine on of course). What you see above is the result of me feeding the MQTT into Node-RED and hence into INFLUX and Grafana on my RPi-based home control.


4 thoughts on “SHELLY EM Contactor Power Control with Dual Power Metering

    1. Not that big – 200 years old solid stone whose insulation ability is as much use as a chocolate fireguard. 3 bedrooms, study, living room, dining room, kitchen and 2 bathrooms (one is small).

      1. Not ideal ground for an ASHP. Hope you’re getting all the RHI subsidies.

        There’s quite a bit on the open energy monitor site about monitoring ASHPs and making sure they are running as efficiently as possible, i.e. not turning on and off all the time…


        1. Hi Simon – as far as I can tell the heating doesn’t turn on and off much less than every half an hour. And yes we are getting the RHI subsidies though with current electricity prices, they aren’t that much use. Who would have thought that well into 21st century we’d be reduced to this. My proposed solution includes closing down our UK property over Christmas next year though it seems that Spanish electricity has taken a hike too.

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