The Shelly Pro EM-50 is a single-phase, two-channel DIN rail energy meter, and it has one feature that sets it apart from the rest of the Pro range: it has a built-in relay. That saves a module for anyone who wants to measure and drive a contactor.
Before opening the board
Switch off at the main breaker and verify the absence of voltage with a multimeter. Not with a neon screwdriver: with a multimeter. If you are not electrically trained, this installation is a job for a professional.
Measure the available space before buying. The module takes a standard rail width, but the clamps are bulky, and on a full board that is the detail that derails the job halfway through.
Power supply
Live to L, neutral to N. It accepts 100 to 260 V and draws under 3 W. Protect it with a type B or C breaker, 16 A maximum.
Torque 0.4 Nm, conductor 0.5 to 2.5 mm², stripped 6 to 7 mm. The torque matters more than it looks: a loose terminal on a device that will sit in a board for years ends up as a poor contact that heats.
The two clamps
Two inputs, IA and IB, up to 50 A per channel. Each clamp goes around a single phase conductor, with the arrow pointing in the direction of current flow towards the load.
If you see negatives at start-up where you expected positives, the clamp is the wrong way round. Nothing needs dismantling: reverse that channel’s measurement direction from the device interface.
The relay, and the mistake that destroys devices
Terminals I and O form a galvanically isolated dry contact rated at a maximum of 2 A at 240 V.
Read that again: two amps. That output drives a contactor coil, not the load. An immersion heater or a pool pump needs an intermediate contactor sized for its power. Wiring the load directly exceeds the contact’s rating and takes the device with it.
It is the most expensive mistake on this installation and the easiest to make, because the device “has a relay” and that sounds like enough.
Network
It supports Wi-Fi, Ethernet and Bluetooth. If you use the network port, always connect or disconnect the cable with the device unpowered: doing it live can damage the port.
On the IP address: if the meter is going to feed an integration — Home Assistant, a car charger, an energy management system — give it a static IP from the outset. A meter that changes address after a router restart stops feeding that integration, and the failure shows up as data that quietly stops arriving.
If the board holds several devices, the SIT-IoT app provisions them as a batch and assigns the static addresses in one pass, rather than going one at a time through each device’s access point.
First start-up
- Restore the supply. The power LED turns red.
- Hold the button for 5 seconds to enable the access point.
- Configure it from the app or by joining the network it broadcasts.
- Check the readings before closing the board. Both channels should give coherent values with the expected sign.
A short press toggles the relay. Ten seconds performs a factory reset, so mind your finger.
The LEDs tell you where the problem is
- Blue: in access point mode, waiting to be configured.
- Red: no Wi-Fi connection.
- Yellow: connected to Wi-Fi but with no route to the cloud. Do not touch the device’s network settings here — the problem is the router.
- Green: all in order.
- Red and blue flashing: firmware updating. Do not cut the power.
What data you get
It measures in four quadrants, so it separates consumption from generation — relevant where there is solar. It records active and apparent power, energy, voltage, current and power factor, at 1% accuracy between 5 and 50 A and 2% below 5 A.
It stores at least 60 days of history at one-minute resolution in non-volatile memory, exportable to CSV and JSON. The internal clock keeps time even if the internet drops, so the history does not lose its order.
A week later, which is when it shows
An installation gets signed off at commissioning, with the installer standing there and everything responding. The problem turns up later: a clamp that springs open with vibration, a device that loses the network when the router changes, firmware that never got updated.
In the console, each device’s last seen and firmware version say at a glance whether the installation is still standing, without going back to the board. And with telemetry refreshing you can check the readings against what you expected without travelling.
The four recurring mistakes
- Clamp around live and neutral together: the fields cancel and the reading drops to almost nothing.
- Load wired straight to the relay: exceeds the 2 A rating and damages the device.
- Clamp lead extended too far: picks up noise and produces phantom watts with everything switched off.
- Network cable connected live: can damage the port.
If the readings do not add up, the causes are the same across the range: they are covered in why a meter reads higher than expected.
The app and the console work with Gen2, Gen3 and Gen4. The app is on Google Play: SIT-IoT Smart Installer Tool.