Smart meters
Smart meters are energy meters fitted at the grid connection point that measure imported and exported power in real time and pass the reading to the inverter over RS485 Modbus RTU, so it can apply export limiting, hold zero feed-in, prioritise self-consumption and decide when to charge a battery. Choose by phase count (single-phase or three-phase), by connection method (direct-connected or via current transformers) and by the meter your inverter's firmware actually supports. Meters from Acrel and Eastron, plus inverter-brand units in accessories. Guide
These meters can record electricity consumption and production at short intervals, often every hour or minute, contributing to more accurate billing, better optimisation of consumption and more efficient use of the energy produced.
With a smart meter, you will know exactly how much energy the PV panels have produced, how much has been consumed and how much has been fed back into the grid. As a PV panel owner, you will be able to optimise the return on your investment.
Smart meters also provide valuable information to utility companies and electricity distributors to help better manage the load on the grid, contributing to a more stable and efficient energy system.
Choose from our range of smart meters, including 3 phase energy meters, and always have an overview of your consumption. We also offer single phase energy meters to fit smaller installations.
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Smart meters: why the inverter cannot regulate without one
An inverter knows exactly how much it is producing, but it has no idea what the house is doing. A smart meter at the grid connection point, wired into the main incomer upstream of every load and of the inverter's own AC connection, closes that gap: it reports bidirectional active power per phase, so the inverter can see the difference between site consumption and generation. That single measurement drives export limiting and zero feed-in, self-consumption control, load diversion to a heating element or wallbox, and the charge and discharge decisions of a battery in a hybrid system.
The meters here communicate over RS485 with Modbus RTU and mount on a standard 35 mm DIN rail. The range covers direct-connected meters for smaller supplies and CT meters that measure through split-core or ring current transformers on larger incomers, in single-phase and three-phase versions, from Acrel and Eastron as well as inverter-brand units sold under the manufacturer's own part number. Before ordering, check three things: the phase count of the supply, the incoming main fuse rating, and the meter list in your inverter's installation manual.
FROM THE FIELD
"Most meter faults we get called out to are not faults at all. The CT is on the right cable but facing the wrong way, or it sits on L1 while the voltage terminal is wired to L3. The meter then reports export while the house is clearly importing, and the inverter throttles itself to nothing. Switch on a kettle, look at the per-phase power sign, and you will find it in two minutes."
Ladislav Proc · Photovoltaics specialist, ONSA Plus
How to choose a smart meter for a PV system
The meter is a small part of the bill and a large part of whether the system behaves. Pick it from the electrical facts of the installation, not from the price list: number of phases, main fuse rating, space left in the consumer unit and the cable route back to the inverter.
Work through it in this order: phase count, then direct-connected or CT against the incomer size, then the RS485 route and termination, then DIN rail width, and last the question that overrides all of them, whether your inverter's firmware supports that exact meter.
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Single-phase or three-phase
Match the meter to the supply, not to the inverter. A single-phase supply takes a single-phase meter with one voltage input and one current path. A three-phase supply needs a three-phase meter measuring L1, L2 and L3 separately, even when the inverter itself is single-phase, because the house load is spread across all three.
- Single-phase inverter on a three-phase supply: still use a three-phase meter. Without it the inverter regulates against one phase and either over-exports or throttles for no reason.
- Summed or per-phase control: where the utility meter nets the three phases, the inverter can regulate on the sum. Where the grid code requires per-phase limiting, the inverter must see each phase individually.
- Neutral is required on almost all of these meters. They measure phase to neutral voltage, so a three-wire connection without N will not work.
Direct-connected or CT, and getting the ratio right
A direct-connected meter carries the full load current through its own terminals, typically up to 63 A, 80 A or 100 A depending on the model. It is the simplest option for a family house: no transformers, no ratio to configure, but the main tails have to be broken and landed on the meter.
- CT meters measure indirectly. The incomer passes through a current transformer and only a small secondary signal reaches the meter, either a 5 A secondary on a ring-type CT or a millivolt output on a split-core clamp.
- Split-core CTs open and clip around the live conductor, so nothing has to be disconnected. That makes them the practical choice for retrofits and for busbars you are not allowed to break.
- The CT ratio must be programmed into the meter or the inverter: a 200/5 A CT is a ratio of 40. Set 100/5 by mistake and every reading is out by a factor of two, which quietly destroys export limiting.
- One CT per measured phase, each enclosing only the phase conductor. If neutral or another live passes through the same window, the currents cancel and the meter reads near zero.
Tip from practice
Commission the meter before the PV is switched on. Switch on a known load of a few kW and confirm each phase shows positive import. Only then energise the inverter and check that surplus shows as negative. Most meters and inverters let you invert a channel in software, so a reversed CT rarely means going back into the consumer unit. Live values are also visible later in monitoring.
CT orientation and phase order
Two mistakes account for nearly all bad readings. The first is orientation: every CT has an arrow, or P1 and P2 markings, that must point in the direction of the grid, from supply towards the load. Reverse it and import is reported as export, so the inverter derates instead of producing.
The second is phase order. The current transformer on L1 has to be paired with the voltage sense on L1. Cross them and the meter multiplies current from one phase by voltage from another, which shifts the apparent power factor and can show a load of several kW as almost no power at all. Label the CT leads before you route them, and verify each phase against a single-phase load rather than trusting the wiring diagram.
RS485, Modbus RTU and termination
The link between meter and inverter is a two-wire RS485 bus running Modbus RTU, with the meter as the slave and the inverter as the master.
- Cable: shielded twisted pair, A to A and B to B, with the shield earthed at one end only. Do not run it in the same conduit as switched or motor circuits.
- Topology: daisy chain from device to device. A star or a long spur causes reflections and intermittent communication faults that look like meter failures.
- Termination: a 120 Ω resistor at each end of the bus. Short indoor runs often work without it, long runs and multi-device buses do not.
- Settings: baud rate, parity and Modbus slave address must match what the inverter expects, commonly 9600 baud and address 1. Every device on one bus needs a unique address.
- Distance: a few hundred metres is realistic at 9600 baud with proper cable. If the meter sits in a distant supply cabinet, plan the route before you buy.
DIN rail width and MID certification
Space in the consumer unit decides more purchases than people expect. Meter width is quoted in DIN modules of 17.5 mm: a single-phase direct meter is typically 1 to 2 modules, a three-phase or CT meter usually 4 modules, around 72 mm. Leave room for the CT secondary terminals and the RS485 gland as well.
MID certification means the meter is approved under the Measuring Instruments Directive 2014/32/EU, annex MI-003 for active electrical energy, and it is what allows the readings to be used for billing between parties: sub-metering tenants, invoicing a shared PV array or charging for energy. It does not turn the meter into a utility settlement device, it does not replace the distributor's own meter at the point of supply, and it does not certify anything beyond the configuration it was approved in. Change the CTs on an MID meter and the approval no longer holds. For selling energy at a public charge point, national rules such as German Eichrecht go further than MID alone.
Matching the meter to the inverter brand
An inverter only talks to the meters in its firmware, because each meter has its own Modbus register map. Fronius, Huawei, GoodWe, Solis, SolaX, SolarEdge and Deye each publish a supported list, and it usually contains their own branded meter plus one or two third-party families such as Acrel or Eastron. A meter that is not on the list may power up and display correctly while the inverter still reports no communication.
- Inverter-brand meters are the safe default: correct registers, correct address, and the commissioning wizard recognises them.
- Acrel and Eastron units are widely referenced in inverter manuals and cover single-phase and three-phase, direct and CT versions.
- Firmware matters: support for a newer meter often arrives in an inverter firmware update, so check the version before ordering.
Quick meter comparison
| Property | What to compare | What it affects |
|---|---|---|
| Phases | Single-phase or three-phase | Must match the supply, not the inverter. Per-phase or summed export limiting |
| Connection | Direct-connected up to 63 to 100 A, or CT | Whether the main tails must be broken, and how large a supply you can measure |
| CT type and ratio | Split-core or ring, 100/5, 200/5, 250/5 A | Ratio has to be programmed. A wrong value scales every reading |
| Communication | RS485 Modbus RTU, baud, parity, address | Cable route, 120 Ω termination, whether the inverter sees the meter |
| Width | 1 to 4 DIN modules of 17.5 mm | Free space in the consumer unit or supply cabinet |
| Certification | MID 2014/32/EU annex MI-003 | Whether readings may be used for billing between parties |
Swipe the table to the left
Frequently asked questions about smart meters
Why does a PV inverter need a smart meter?
Because the inverter can only see its own output, not the household load. A meter at the grid connection point reports import and export in real time, which is what makes export limiting, zero feed-in, self-consumption control and battery charge decisions possible.
Where exactly is the smart meter installed?
At the grid connection point, on the main incomer downstream of the utility meter and upstream of every load and of the inverter's AC connection. Fitted anywhere else it sees only part of the site and the regulation will be wrong.
Do I need a single-phase or a three-phase meter?
Match the meter to the supply. A three-phase supply needs a three-phase meter even if the inverter is single-phase, because the load is spread across L1, L2 and L3. Only a genuinely single-phase supply takes a single-phase meter.
What is the difference between a direct-connected meter and a CT meter?
A direct-connected meter carries the full current through its terminals, usually up to 63 to 100 A, so the tails must be broken and landed on it. A CT meter measures indirectly through current transformers, which suits larger incomers and retrofits.
How do I set the CT ratio?
Read it off the transformer and enter it in the meter or inverter menu. A 200/5 A CT is a ratio of 40. Any mismatch scales every reading proportionally, so a wrong ratio makes export limiting under-react or over-react by that factor.
Which way round should the current transformers face?
The arrow, or the P1 to P2 marking, points from the grid towards the load. Reversed, the meter reports import as export and the inverter derates. Verify by switching on a known load and confirming the phase shows positive import.
Does the phase order of the CTs matter?
Yes. The CT on L1 must be paired with the voltage sense on L1. Crossed phases multiply current from one phase by voltage from another, which distorts the power factor and can show a multi-kW load as almost zero power.
Do I need a 120 ohm terminating resistor on the RS485 line?
On long runs and multi-device buses, yes: one 120 Ω resistor at each end. Short indoor links often work without it. Always use shielded twisted pair, wire A to A and B to B, and daisy chain rather than star the bus.
Can I use any Modbus meter with any inverter?
No. Each meter has its own Modbus register map and the inverter only reads the ones in its firmware. Check the supported meter list in the inverter manual, and note that support for newer meters often arrives with a firmware update.
What does MID certification actually authorise?
MID under directive 2014/32/EU, annex MI-003, allows the readings to be used for billing between parties, for example sub-metering tenants. It does not replace the distributor's settlement meter, and the approval covers only the configuration certified, including its CTs.
Meter model, wiring and CT ratio must match the specific inverter and its firmware version, and metering for billing is subject to national rules.
About ONSA Plus
Why installers across Europe order meters from us
ONSA Plus is a European B2B distributor of photovoltaics, battery storage, heat pumps and EV charging stations. In our own EU warehouses we keep over 1,500 products from 25+ brands, including Huawei, Solis, SolaX and Dyness.
We are not just a trading company. We design and service energy systems ourselves, so we answer questions on meter wiring, CT sizing and inverter pairing from engineering practice, not from a catalogue. You handle a warranty claim directly with us and you buy per job.
