Quick Summary (TL;DR)
Choose Shelly Pro 3EM if you need to measure a three-phase grid connection, want Ethernet and DIN-rail mounting, or need three independent single-phase measurement positions. The commonly marketed Pro 3EM 120A v2 comes with three 120 A current transformers (CTs), provides separate phase measurements and has approximately 60 days of minute-resolution history on the device. It also supports local network integrations, including the native Home Assistant Shelly integration.
Choose Shelly EM Gen3 if you have a single-phase installation and only need one or two measurements: for example, overall household load plus a dedicated heat-pump circuit, or grid exchange plus a separate solar circuit. It has two CT inputs, Wi-Fi rather than Ethernet, up to 10 days of minute-resolution local records, and a dry-contact output intended for appropriate external contactor control. The standard EM Gen3 pack currently includes one 50 A CT, not automatically two clamps. Check the exact bundle before pricing the installation.
Do not confuse Shelly EM Gen3 with the older first-generation Shelly EM. They share a similar name and two-channel concept but differ in firmware platform, documented interfaces and clamp options. Neither is a substitute for true three-phase phase-by-phase monitoring simply because it has multiple CT sockets. A qualified electrician should confirm voltage references, clamp rating, enclosure space and all protective arrangements before installation.
Shelly Pro 3EM vs Shelly EM Gen3: specification comparison
This comparison uses the current Shelly EM Gen3 and the Pro 3EM 120A v2 as the main reference models. The Pro family also has different CT packages (including 400 A and specialised 63 A variants), while Shelly still sells a legacy EM. Product names and retailer bundles alone are therefore insufficient to establish capability.
| Feature | Shelly EM Gen3 | Shelly Pro 3EM 120A v2 |
|---|---|---|
| Primary purpose | Two measurements on one AC phase | Three-phase meter or up to three separate single-phase measurement points |
| Current transformerAliExpress price inputs | 2 | 3 main measurement inputs; optional neutral monitoring provision depends on exact version |
| Clamp package | Manufacturer EU offer includes one 50 A CT; second CT may cost extra | Manufacturer specifies three 120 A CTs included for the 120 A v2 bundle |
| Compatible CT range | Manufacturer lists its 50 A and 80 A CTs for EM Gen3 | Match the official CT rating and exact Pro 3EM hardware SKU (120 A, 400 A or another variant) |
| Three-phase metering | No – one voltage reference for two single-phase channels | Yes – phase-by-phase voltage and current references |
| Network | 2.4 GHz Wi-Fi; Bluetooth is present but not a substitute for Ethernet | Ethernet/LAN, Wi-Fi and Bluetooth |
| Mounting | Compact unit for suitable protected electrical locations | DIN-rail electrical distribution cabinet |
| Local minute-resolution log | At least 10 days according to Shelly | Up to 60 days according to Shelly |
| Native Home Assistant integration | Yes, supported by Home Assistant Shelly integration | Yes, supported by Home Assistant Shelly integration |
| Control output | Built-in dry-contact relay for suitable external-contactor control; 2 A output rating | No integrated load relay; optional switch add-on for external-contactor use |
| Grid import/export | Check configured direction and available cumulative energy entities; two channels alone do not establish a three-phase total | Four-quadrant measurement with per-phase import/export monitoring |
| Best fit | One-phase grid or selected circuits, compact retrofit | Three-phase incoming supply, robust wired networking, broader metering plans |
Important: these are manufacturer-described capabilities, not independent lab measurements. Pro 3EM v2 marketing mentions a Matter upgrade path; do not base a Home Assistant purchasing decision on unverified firmware-dependent features when the native local Shelly integration already exists.
The fundamental difference: two channels are not the same as three phases
A current transformerAliExpress price surrounds one conductor and measures current. The device also needs a relevant AC voltage reference to calculate real power and accumulated energy. The relationship between each CT and its voltage reference matters; a clamp count alone cannot tell you whether a unit can measure a whole property accurately.
Single-phase household: what two channels really buy you
In a normal single-phase service, one EM Gen3 can be a sensible starting point. One CT might observe the incoming live conductor while the second measures a selected subcircuit such as an immersion heater or a heat pump. Both readings share the device’s single-phase measurement design, making this a natural use case if the monitored conductors really are on the same phase.
The two channels should not simply be added together to produce “whole-house consumption” when one CT covers the total incoming circuit and the other is a subset of it: that would count the subset twice. Treat the main CT as the supply boundary; use the other as supplementary appliance visibility. If the second CT watches a solar inverter conductor, identify whether its sign and cumulative energy data represent production, feed-in, or only circuit flow before assigning it in Home Assistant.
Three-phase household: why Pro 3EM changes the answer
For a three-phase grid connection, accurate site-level reporting needs the current and voltage relationship for each phase. Shelly Pro 3EM is engineered for this, with three phase channels and monitoring intended for import/export installations. An EM Gen3 does not become a three-phase meter by fitting two clamps to different phases. That leaves missing phase information and may associate measured current with the wrong voltage phase.
Where Pro 3EM is used to monitor three distinct single-phase circuits rather than one three-phase incomer, an electrician must follow the manufacturer’s documented single-phase connection scheme. Even with three channels, it is not a 16-circuit panel monitor. Measuring a whole-house incomer and separately measuring several appliances generally requires additional hardware.
Which Shelly EM? Gen3 vs the original Shelly EM
Listings headed “Shelly EM” may mean the older legacy product, not Shelly EM Gen3. The first-generation EM also provides two single-phase channels and contactor control, but it belongs to Shelly’s older device platform. Current vendor listings for the old EM offer packages with 50 A and 120 A clamps, while EM Gen3 documentation explicitly lists 50 A and 80 A compatible CTs. You should not assume old and new clamps are interchangeable, or buy a 120 A legacy accessory for a Gen3 device without vendor confirmation.
| Purchasing check | Original Shelly EM | Shelly EM Gen3 |
|---|---|---|
| Device identification | Legacy Shelly EM; check exact seller SKU | Model S3EM-002CXCEU |
| Architecture | First-generation Shelly platform | Gen3 platform with Shelly RPC interfaces |
| Nominal monitoring scope | Two single-phase measurements | Two single-phase measurements |
| Clamp option example | Legacy manufacturer bundles include 50 A / 120 A variants | Official KB specifies 50 A / 80 A compatible CTs |
| Home Assistant connection | Native Shelly integration supports Gen1; CoIoT configuration can matter | Native Shelly integration supports Gen3 via newer RPC communications |
| Local history | Confirm against exact old revision/firmware | At least 10 days, 1-minute resolution |
| Buying advice | Consider only when the discount or exact clamp requirement justifies legacy hardware | Default choice for a new compact two-channel install |
A seller calling a unit “Shelly EM 120A” is not evidence that it is EM Gen3. Verify the label and the official product sheet, particularly when mixing second-hand units, CTs and mounting accessories.
What the meters actually measure
Shelly’s monitoring features go well beyond an on/off load indicator, but the words power, energy and apparent power have different meanings. Power (W) is the instantaneous rate of electrical use or generation. Energy (kWh) is accumulated use over a period; 1 kW held for one hour amounts to 1 kWh. Apparent power (VA) and power factor can help explain motor and electronic loads but should not be mistaken for billable active energy.
EM Gen3 reports active/apparent power and energy, power factor and related measurements. The Pro 3EM family is more explicitly designed for per-phase power and four-quadrant energy flows, especially relevant to three-phase supplies and solar export. An energy meterAliExpress price in the consumer unit is still not automatically a regulated, utility-certified billing meter or a substitute for the supplier’s meter. Shelly’s stated accuracy figures are model- and measurement-specific.
How much accuracy can you expect?
Shelly describes Pro 3EM active energy as approximately 1% accuracy under its specified conditions and cites Class B active-energy testing. For EM Gen3, the official technical specification gives ±2% current-measurement accuracy over 1–50 A and ±5% below 1 A, together with separate voltage accuracy information. These numbers are not directly interchangeable: a current accuracy figure is not a complete guarantee of total energy accuracy at every load and power factor.
At low loads, CT systems may show higher relative errors or enforce no-load thresholds. Shelly documents a 30 VA-per-channel no-load threshold for EM Gen3; the Pro 3EM family also documents model-specific low-load behaviour. This matters for measuring an always-on router at only a few watts. For such loads, a suitable appliance-level meter or smart plugAliExpress price may be more informative than a high-range main-service CT.
Current transformers: rating, quantity and physical fit
The CT is not a generic accessory. Its output characteristics, ratio and burden requirements must match the meter electronics. Start from the actual maximum current on the conductor, plus anticipated headroom, rather than from a seller’s description of the device as “whole-home”. Then confirm the clamp’s inner aperture fits the specific conductor with insulation, that the housing can be safely closed, and that the electrician can route sensor leads properly in the enclosure.
| Question to ask | EM Gen3 | Pro 3EM 120A v2 |
|---|---|---|
| How many CTs are needed? | One for one channel; two for two channels | Three to capture three phase conductors |
| What is included? | One 50 A CT in the referenced current official EU bundle | Three 120 A CTs in the referenced v2 bundle |
| How much current must be supported? | Check against 50 A or officially supported 80 A CT hardware | Check actual supply rating against 120 A CTs and specific model |
| Can I reuse other CT brands or the old EM 120 A clamp? | Do not assume compatibility | Do not mix CT ratios or 400 A and 120 A SKUs without approval |
| Will it fit the cabinet? | Compact module still needs protected supply and CT space | DIN-rail width, cable bending and CT positions need allowance |
If the incomer is rated above the supported measurement range of the candidate device, do not under-size the CT to save money. Likewise, an extremely oversized CT is a poor choice for a very small load where low-current sensitivity matters. Our ESP32 CT clamp energy monitor guide explains the general sensing approach; it is a DIY overview, not a wiring manual for a live consumer unit.
Internet, Wi-Fi and Ethernet: which is more dependable?
Both meters can communicate locally to Home Assistant using the official Shelly integration; Shelly Cloud is not required for the native Home Assistant connection. For newer Shelly generations, Home Assistant uses local RPC communication and supports device discovery and manual IP configuration. An internet outage therefore need not remove local monitoring, provided local power, the device and LAN remain functional.
The key physical difference is network transport. EM Gen3 uses Wi-Fi, so reception and access-point uptime determine the quality of its connection. Pro 3EM adds a wired LAN port, which is often attractive inside a metal distribution enclosure with difficult radio coverage. However, Ethernet is not a cure for incorrect voltage/CT configuration, and it still relies on working network infrastructure. The appropriate design might include a nearby switch or a suitable access point.
If privacy or continuity matters, test the setup with the router’s external internet access blocked while leaving the local LAN running. Confirm that power sensors, cumulative energy statistics and Home Assistant automations still update. Distinguish Home Assistant’s own retained long-term statistics from short-lived device logs: 10 or 60 days of device-side minute samples does not limit how long a properly configured Home Assistant database can retain energy statistics.
Does Shelly Pro 3EM support Modbus and automation?
Shelly documents Modbus functionality for the Pro 3EM family, along with MQTT, HTTP, WebSocket and scripts. This can matter when connecting to an industrial controller, a local energy management system or a compatible supervisory system. It does not mean that every Pro 3EM protocol feature is present on EM Gen3: the latter’s documentation lists MQTT, HTTP, WebSocket and RPC, not an equivalent published Modbus feature for this model.
For a normal Home Assistant installation, you usually do not need to manually construct Modbus register mappings for either Shelly product. The native Shelly integration is simpler, has its own supported entity model and avoids the maintenance burden of a custom polling configuration. Use a direct Modbus path when a specific external system actually requires it and check the exact Pro 3EM firmware and published register/protocol support.
Can either unit switch the measured load?
This is a particularly important purchasing trap. Measuring a circuit is not the same as switching its supply. EM Gen3 includes a dry-contact output rated by the manufacturer for up to 2 A at 240 V AC; its purpose is suitable external contactor control or another circuit within the specified rating. It is not a high-current 50 A or 80 A load-breaking switch simply because the CT can measure that much current.
Shelly Pro 3EM does not have a built-in power relay for the monitored conductors. Where supported by the exact device revision, its optional switch add-on permits external contactor integration. Any load-shedding or EV/heat-pump control must preserve protective devices, approved interlocks, equipment warranties and local electrical codes. Do not rely on a cloud automation or Wi-Fi connection as a safety-critical protective function.
Home Assistant Energy dashboard: use the right entities
The Home Assistant Energy dashboard needs energy statistics suitable for the boundary you are measuring. A rapidly changing sensor in watts is useful for live flow cards and load-shedding logic; a cumulative sensor in kWh is normally the relevant input for historical grid consumption. Do not blindly select an entity simply because its name contains “power” or “total”. The exact entity list may differ with firmware, Shelly model and Home Assistant release.
- Integrate locally: put the Shelly device and Home Assistant on an appropriately routed LAN; use Settings → Devices & services → Shelly and confirm that expected sensors appear.
- Verify one measurement at a time: compare an observed electrical load against device power readings and check the sign of net grid flow. Ask the installer to investigate suspicious or reversed values rather than reorienting live clamps yourself.
- Identify cumulative import/export readings: choose the appropriate increasing energy entities for grid import and, if fitted with solar or bidirectional supply, export. Do not use a net signed energy register as though it were a monotonic import counter.
- Choose the correct boundary: a main CT plus a monitored heat-pump branch is still one grid source plus a separate device, not two household sources. A solar production CT does not automatically represent total solar generation if the installation contains batteries or multiple inverter paths.
- Configure the Energy dashboard: go to Settings → Dashboards → Energy and add the correct grid-connection energy sources. If no suitable entity appears, examine its availability, units, device class, state class and Recorder history.
- Validate over at least a day: compare directional totals to the supplier meter, watch for negative or implausible values and confirm the data survives ordinary router or Home Assistant restarts.
Home Assistant provides official guidance on electricity grid integration. For ongoing sensor and statistics problems, see our Energy Dashboard troubleshooting guide. If you are still choosing between CT, PZEM, pulse counting or Modbus monitoring, the ESP32 energy monitoring methods comparison can help decide whether Shelly is the right architecture at all.
Solar PV and grid import/export: common measurement mistakes
For an ordinary grid-tied solar property, “how much the house uses”, “how much the panels make”, “how much is bought from the grid” and “how much is exported” are different measurements. A correctly placed bidirectional meter at the grid connection measures grid exchange; it does not directly measure PV generation behind that point. If you want a complete solar self-consumption picture, combine grid data with suitable inverter production and, where relevant, battery charge/discharge data.
A single-phase solar installation with an EM Gen3 might use one CT for grid exchange and the other for a dedicated inverter feed, if the electrical topology and measurement configuration permit. That does not automatically cover a multi-phase property or complex hybrid inverter wiring. For a three-phase service, Pro 3EM is generally the more appropriate Shelly starting point because import and export can occur on different phases. Your utility’s settlement method (per-phase or summed netting) may differ from an individual meter’s displayed quantities, so interpret the figures accordingly.
Import and export also should not be approximated by subtracting cumulative counters after the fact. A net total over a day can hide times when power flowed in opposite directions. Confirm the separate directional statistics required by Home Assistant before you commit to a monitoring scheme. For the separate topic of solar integration, link to a dedicated solar article only when it is actually published.
Installation realities: space, mains safety and Wi-Fi placement
Although CT monitoring avoids routing the entire monitored current through the measurement electronics, neither Shelly unit is a plug-in consumer gadget. Both require an appropriate voltage/supply connection, and installing CTs generally means opening an electrical cabinet. A qualified electrician should carry out the work in accordance with local requirements, including the correct protection, phase relationship, separation of cables and suitable enclosure arrangement. In the UK, work may involve BS 7671 and Part P obligations where applicable; elsewhere, including EU states, local rules apply.
- Space: EM Gen3 is physically compact, but two clamps and their leads still need room. Pro 3EM needs DIN-rail capacity and usable access to three phase conductors.
- Supply and measurement references: correct phases and voltage sense connections are essential for real-power readings. Labelled CT direction and device settings must be verified at commissioning.
- Network: verify real Wi-Fi conditions with the electrical cabinet closed, not just with its metal door open. Consider wired LAN for Pro 3EM where the cable route is practical.
- Serviceability: allow for future electrician access, device replacement, cable strain relief and making notes of CT placement and circuit labels.
- Safety and use limitations: never open a live distribution board, move CTs around live conductors or bypass the installation’s protective devices as a DIY calibration experiment.
Real total cost: the monitor is only one line item
The least expensive device is not necessarily the least expensive installed system. Price comparisons based only on a Shelly product page can miss a second CT, appropriate enclosure space, a DIN-rail extension, a cable route for Ethernet, commissioning and electrical labour. Prices, VAT treatment, warranty and shipping differ across countries and retailer bundles; they also change over time. Rather than freezing a potentially outdated price in this article, compare your actual local quotations against the parts list below.
| Cost element | EM Gen3 project | Pro 3EM project |
|---|---|---|
| Meter hardware | One compact two-channel unit | One DIN-rail three-channel unit |
| CT sensors | Check bundled first 50 A CT; add suitable second CT if needed | Check three included 120 A CTs on v2 bundle |
| Panel fittings | Suitable protected mounting and cable route | DIN-rail space, protection and conductor/CT access |
| Networking | Wi-Fi coverage, possible access-point improvement | Ethernet cable/switch or Wi-Fi coverage |
| Electrician time | Circuit identification, voltage reference, protected connection, commissioning | Three-phase identification, CT orientation per phase, commissioning |
| Control expansion | External contactor and associated protection if switching needed | Compatible optional switch add-on, external contactor and protection if needed |
| Data platform | Home Assistant host and backups | Home Assistant host and backups |
| Ongoing expenses | No Shelly Cloud subscription required for native local HA; any optional service costs are separate | Likewise no Shelly Cloud subscription required for native local HA; any optional service costs are separate |
Illustrative arithmetic, not a retail quote: if one unit costs £65 but needs a £20 second clamp and £120 in electrical labour, the installed project is £205 before miscellaneous materials. A £150 unit with all required CTs that costs £150 to install totals £300. Those are hypothetical numbers to demonstrate the calculation, not verified product prices or installation quotations. If a three-phase home would need multiple smaller devices, the correct Pro unit may avoid a more costly or electrically unsound workaround.
Which one should you buy? Four practical scenarios
A single-phase flat, monitoring the incoming feed
Buy EM Gen3 if its officially supported clamp covers your supply and the electrician confirms the device can be installed appropriately. The second input can remain unused or monitor one significant branch circuit. Pro 3EM is functional in supported single-phase configurations but usually brings more features and size than this simple job needs.
A single-phase house with a heat pump or immersion heater
Prefer EM Gen3 if one channel can measure overall grid exchange and the second can measure the chosen branch. The extra data tells you whether your heating equipment is the principal contributor to peaks. Avoid adding the branch’s energy a second time in the Energy dashboard when it is already included in the total.
A three-phase home with PV and possibly an EV charger
Choose Pro 3EM as the better Shelly foundation for phase-by-phase grid measurements. However, an EV charger, PV inverter or home battery may require separate approved telemetry to produce a complete energy balance. Three Pro channels at the incomer are not the same thing as dedicated EV and solar circuit submetering.
A workshop or business needing wired, local integration
Choose Pro 3EM when its DIN-rail form, Ethernet and documented industrial-oriented interfaces fit your control system. Verify the exact CT rating against the installation and any local requirements for certified or revenue-grade metering. A general-purpose smart energy monitor should not be represented as a legal billing instrument.
Where neither Shelly is the right choice
If you want 12–16 separately identified circuits, two- or three-input meters will not give you that circuit density without multiple devices. Consider a purpose-built multi-circuit solution, subject to regional support and Home Assistant integration requirements. If wired fieldbus and electrical-engineering compatibility dominate, a suitable Modbus electricity meter may be a better fit. Where you only need one plug-in appliance, a correctly rated smart plugAliExpress price with energy measurement is simpler than opening a consumer unit.
Checklist before ordering
- Supply: is the site single phase or three phase, and which circuit or electrical boundary is to be measured?
- Exact product: is the listing Shelly EM (legacy), EM Gen3, Pro 3EM 120A v2, Pro 3EM-400 or another specific variant?
- CT inventory: how many compatible clamps are included, which ratings are approved, and will they fit the conductors?
- Monitoring scope: do you need net grid exchange, per-phase import/export, a solar feed or particular subcircuits?
- Cabinet: is there proper protected space for the module, CTs, wiring and maintenance access?
- Network: is stable 2.4 GHz Wi-Fi available, or is wired Ethernet strongly preferable?
- Home Assistant: are native local Shelly data, persistent energy counters and an appropriately retained Recorder configuration sufficient?
- Control: is relay/contactor switching actually required, and does the planned solution comply with equipment and electrical safety limits?
- Quotation: does the electrician’s quote include commissioning, sign checks and verification against existing utility readings?
Frequently asked questions
Can Shelly EM Gen3 monitor a three-phase supply with two CTs?
No. Shelly documents EM Gen3 as a single-phase, two-channel energy meter. Having two current sensors does not provide three phase-referenced measurements. Use hardware explicitly designed for the installation, such as an appropriate Pro 3EM configuration.
Can Pro 3EM monitor three separate circuits on single-phase power?
Yes, Shelly documents supported single-phase arrangements with multiple measurement positions in the Pro 3EM family. This requires the correct manufacturer wiring scheme and installation by a qualified electrician. It still gives three measured positions, not an unlimited number of appliances.
Does Shelly EM Gen3 include two clamps?
Not necessarily. The referenced manufacturer EM Gen3 50 A package explicitly includes one 50 A clamp; the device has two inputs. Check the retailer’s exact bundle contents before assuming a second compatible clamp is supplied.
Does a Shelly meter work with Home Assistant without the cloud?
Yes. Home Assistant documents direct local communication for its Shelly integration and states that Shelly Cloud is not required. You still need a functioning local network, correct device configuration and a running Home Assistant server.
Can I use the Shelly EM Gen3 relay to switch a whole house?
No. The device’s small dry-contact relay is not a high-current main service switch. Any external contactor or load-control application needs appropriate engineering, protection and a qualified installer. The clamp’s ampere rating does not become the relay’s breaking capacity.
Should I pick a 120 A clamp for a small 10 A circuit?
Choose the CT range recommended for the actual circuit and device, not the biggest number available. Some devices offer more than one officially supported range, but low-current accuracy and compatible CT electronics matter. Use only the CTs approved for your exact device.
Final recommendation
For a straightforward, single-phase monitoring project, the Shelly EM Gen3 is the more targeted purchase. It is compact, locally integrable and provides two independently useful measurement positions, provided that the supplied clamp and supported current range suit the circuits. For true three-phase measurement, Ethernet, per-phase grid visibility or a DIN-rail installation, choose the Shelly Pro 3EM. The 120A v2 bundle’s three included CTs can also change the total project cost calculation. Make your choice around the electrical topology and installed bill of materials—not the similarity of the product names.
Official sources and further reading
- Shelly Knowledge Base — Shelly EM Gen3: specifications, 50 A/80 A CT compatibility, measurement data, outputs
- Shelly — EM Gen3 product and package contents
- Shelly — Pro 3EM 120A v2 product information, supplied CTs and features
- Shelly — Pro 3EM-400 variant and local interface description
- Shelly Knowledge Base — original Shelly EM specification (legacy model)
- Home Assistant — official Shelly integration, local RPC and Gen1/Gen3 differences
- Home Assistant — Integrating your electricity grid
- Home Assistant — Energy management FAQ
Editorial notes: Checked against manufacturer and Home Assistant references in October 2026. This is a specification- and documentation-based comparison, not a hands-on product review. Product revision, CT bundle, warranty, firmware, regional electrical suitability and retailer pricing can change; confirm against the exact SKU before ordering. Always use an appropriately qualified electrician for cabinet work.