Shelly Pro 3EM-400

The maximum number of circuits the device can measure at the same time, counting both the mains and the individual branch circuits.

Two channels only tell you what the whole house is doing. Sixteen or eighteen channels let you see which breaker feeds the water heater, the EV charger or the always-on standby load.

If you need more channels than a single unit provides, some brands let you install a second device and merge both in the same app.

How many current transformer (CT) clamps come in the box.

The same monitor is often sold in several kits: mains only, 8 sensors or 16 sensors. Extra clamps can usually be bought separately, so check what you actually get before comparing prices.

The current rating of the clamps that go around the service mains, expressed in amps.

The clamp has to be rated above your main breaker: a 200 A service needs 200 A CTs, while a European 63 A or 120 A supply is fine with smaller ones. An undersized clamp saturates and under-reports during peaks; a heavily oversized one loses precision on small loads.

The current rating of the smaller clamps used on individual breaker circuits, typically 50 A, 60 A or 80 A.

Most household circuits are 15 A to 50 A, so almost any branch clamp works. It matters for big loads such as an EV charger, a heat pump or an electric range, where a 50 A clamp can be too small.

How close the readings are to reality, given either as a percentage error (for example +/- 1% or +/- 2%) or as an IEC accuracy class.

Consumer clamp monitors sit around +/- 1% to +/- 2%, which is plenty for spotting energy hogs and comparing months. Billing tenants or validating a solar contract asks for a tighter class, and no clamp-based device is a legal replacement for the utility meter.

Support for the split-phase 120/240 V service used in North American homes, where two hot legs share a neutral.

A split-phase monitor measures both legs so that 240 V appliances such as dryers, ovens and EV chargers are reported correctly instead of being counted once.

The ability to measure a three-phase supply, with one voltage reference and one current clamp per phase.

Three-phase service is common in European homes, workshops and anything with heavy machinery. A monitor built only for single-phase or split-phase installations cannot reconstruct the totals correctly on such a panel.

Bidirectional measurement, which tells energy coming from the grid apart from energy exported back to it.

Without it, a solar array simply makes the reading go down or turns it into a meaningless absolute value. With it you get consumption, production, self-consumption and the surplus you are selling, which is what makes automations like running the dishwasher on excess solar possible.

Reporting of power factor, and usually of voltage, current and reactive power alongside the plain watt-hours.

Power factor reveals how much of the current actually does work. It is what separates a basic consumption tracker from a device you can use to diagnose motors, inverters and poor wiring.

The device is installed inside the electrical panel, with the clamps closed around the mains and the branch circuits.

This is the most complete way to measure a home, but it means working next to live busbars. Many regions require an electrician, and doing it yourself can affect your insurance.

The housing clips onto the standard 35 mm DIN rail found in European consumer units and in commercial panels.

DIN mounting keeps the meter tidy and serviceable, and it is often the only accepted way to install a device in a panel that gets inspected.

The monitor reads the utility meter directly through its data port (P1 in the Netherlands and Belgium, DSMR telegrams, or an optical eye) instead of using clamps.

Reading the meter is a five minute plug-in job with no panel work and gives you exactly the numbers you are billed on, including the export register. The trade-off is that you see the house total only, with no per-circuit detail.

How the monitor itself is powered: from the mains inside the panel, from a plug-in adapter, over USB, or from the meter port it is plugged into.

Panel-powered units need a spare breaker or a set of voltage taps, while USB and meter-port devices only need a socket nearby.

Range: The 2.4 GHz band offers a longer range, meaning it can cover larger areas and penetrate solid objects like walls more effectively than 5 GHz.

Speed: Generally, 2.4 GHz provides slower speeds compared to 5 GHz due to a lower maximum data rate.

Interference: More prone to interference since many other devices (like cordless phones, microwaves, and Bluetooth devices) operate on the 2.4 GHz frequency, leading to potential congestion and slower network performance.

A metal breaker panel blocks a lot of signal, which is why the longer reach of 2.4 GHz is the norm in this category.

Range: Offers a shorter range compared to 2.4 GHz and may have difficulty penetrating walls and other obstacles.

Speed: Typically provides faster speeds and higher data rates due to more available bandwidth.

Interference: Less prone to interference as fewer devices operate on this frequency, leading to a more stable connection.

Ethernet is a widely used technology for wired networking, providing a stable and high-speed connection between devices on a local area network (LAN).

Stability and Reliability: Because it uses a wired connection, Ethernet is less susceptible to interference from other devices and physical barriers, leading to a more stable and reliable connection.

For a meter locked inside a metal enclosure, a wired port is often the difference between continuous data and a graph full of gaps.

Bluetooth is a wireless communication technology used for exchanging data over short distances.

In energy monitors Bluetooth is mostly used for the initial setup and for local configuration when the WiFi link is down, rather than for day to day data.

Z-Wave is known for its low power consumption, making it ideal for battery-powered devices. It supports a mesh networking topology, allowing devices to communicate with each other directly or through intermediary nodes, thus extending the overall network range and enhancing reliability. The protocol supports data rates of up to 100 kbps, suitable for transmitting control commands and sensor data.

A Z-Wave meter reports to your hub instead of to a cloud account, which keeps the data inside the same system that runs your automations. It needs a Z-Wave hub or controller.

Zigbee is a wireless communication protocol designed for low-power, short-range applications, commonly used in smart home devices, industrial automation, and other areas requiring reliable, low-data rate communication. It features low power consumption, supports short-range communication and employs mesh networking, allowing devices to relay data through each other to extend range and improve reliability.

Zigbee devices need a Zigbee-compatible hub to connect and communicate with other devices in your smart home setup.

Thread is a wireless communication protocol designed specifically for Internet of Things (IoT) devices, providing secure, reliable, and scalable networking. It is IP-based, operates in the 2.4 GHz band and uses mesh networking to enhance reliability and extend range by allowing devices to relay data through each other.

Thread-enabled devices require a Thread Border Router (such as certain smart home hubs or routers) to connect to your home network and other Thread devices.

Additional connectivity options not covered by the standard protocols.

This may include proprietary sub-GHz radios, RS-485 and Modbus links, or the wired bus some manufacturers use between the meter and its display.

Google Home serves as a central hub for a variety of smart devices, including lights, thermostats, cameras, and speakers. It leverages Google Assistant for voice control, enabling users to manage their smart home with simple commands.

Amazon Alexa is one of the most widely used smart home systems, known for its compatibility with a vast array of devices. Users can control everything from smart lights to appliances using voice commands through Echo devices.

Apple HomeKit is designed for users in the Apple ecosystem, providing a seamless way to control compatible smart devices through iOS devices using the Home app or Siri. It emphasizes security and user privacy.

SmartThings is a flexible platform that connects a wide range of devices from different brands, offering a unified interface for control. It supports various communication protocols, allowing for extensive device compatibility.

Home Assistant is an open-source platform that allows for extensive customization and integration of a wide variety of smart devices. It supports a multitude of protocols and brands, making it ideal for tech-savvy users.

Its Energy dashboard is the main reason many buyers pick one monitor over another: it takes grid consumption, return to grid and solar production and turns them into long term statistics without any cloud service.

Matter is a unified, open-source connectivity protocol designed to standardize and simplify communication between smart home devices from different manufacturers. Developed by the Connectivity Standards Alliance, Matter aims to improve interoperability, security, and reliability across various smart home ecosystems.

Matter operates over existing networking technologies, including Ethernet, Wi-Fi, and Thread, providing flexibility in device connectivity. Support for energy measurement device types arrived later than for lights and plugs, so a Matter-certified meter is still the exception rather than the rule.

How often the device publishes a new reading, from one second down to one sample per minute.

Second by second data is what lets you turn an appliance on and watch its signature appear on the graph. Slower reporting is fine for monthly totals but useless for diagnosing short spikes or for automations that react to surplus solar.

How long the device or its app keeps your measurements, and at what resolution.

Vendors usually keep high resolution data for hours or days and then roll it up into minutes, hours and days for months or years. Long retention is what makes year over year comparisons and before and after checks possible.

The app converts kilowatt-hours into money using the electricity rate you enter.

Seeing a figure in your own currency instead of kWh is what usually drives behaviour change, and it makes the estimated monthly bill comparable with the one the utility sends.

Support for tariffs where the price changes by hour, day or season, including peak and off-peak windows.

If you are on a time-of-use or dynamic plan, a monitor with a flat rate setting will misprice everything. This is also what lets the app tell you to shift the dryer or the EV charge to the cheap window.

The ability to download your readings, usually as a CSV file or through a documented download endpoint.

Export keeps your history usable outside the vendor's app, which matters if the company shuts the service down or if you want to build your own spreadsheets and reports.

Notifications triggered by consumption thresholds, unusual activity or a circuit that has been drawing power for too long.

Alerts turn a passive dashboard into something that reaches you: an oven left on, a sump pump running non-stop, or a monthly budget about to be blown.

A documented interface (REST, JSON-RPC or similar) that serves the readings straight from the device on your own network, with no cloud account in the middle.

A local API means your data keeps flowing if the internet or the vendor's servers go down, and it is what makes the difference between a monitor you can build on and one you can only look at.

The device can publish its measurements to an MQTT broker on your network.

MQTT is the lingua franca of self-hosted automation: anything subscribed to the topic (Home Assistant, Node-RED, InfluxDB, a custom dashboard) gets the values pushed the moment they change.

A web page served by the device itself, reachable through its IP address, for configuration and live readings.

A built-in web UI lets you commission the unit, calibrate the clamps and check the numbers without installing an app or creating an account.

A built-in switching output, or an add-on contactor control, that lets the meter cut or enable a load by itself.

It turns the monitor into an actuator: shed a load when you approach the contracted power limit, or switch on the water heater while the solar array is exporting.

Built-in logic for peak demand control, load shedding or excess solar diversion, driven by the readings the device takes.

Instead of only reporting, the platform acts: it delays an EV charge until the price drops, keeps the house below a demand threshold, or sends surplus production to a specific appliance.

The ability to create automated actions based on sensor readings or schedules.

Automations can trigger other smart devices, send notifications, or adjust settings without manual intervention.

It keeps working, and can be controlled from inside your own network, when the internet is down: over Zigbee, Z-Wave or Thread through a local hub, a native Matter or HomeKit connection, a documented local API or an official local integration.

A product without local control stops answering during an internet outage and depends on the maker's servers staying online.