Choosing an Energy Meter: Direct or CT, Modbus or Wi-Fi

Most energy-meter mistakes are made before the meter is bought: a single-phase meter on a three-phase board, a direct-connect meter on a feeder that carries 150 A, a Wi-Fi meter inside a closed steel cabinet, or a meter chosen for export limiting that the inverter cannot read. This guide walks through the four choices in the order they should be made — connection, phases, accuracy class and communication — and then looks at one special case, photovoltaic export limiting.

The figures quoted for AgControlli meters are the published specifications. Where conformity is mentioned it is as stated by the manufacturer, and certificates are confirmed before order.

1. Direct connection or current transformer?

A direct-connected meter passes the load current through the meter itself. It is the simplest and cheapest option, and it is limited by the meter’s current rating: the AGL200, for example, is rated 10(80) A, so it suits circuits up to 80 A. Above that, or wherever opening the conductor is not acceptable, the meter needs a current transformer (CT).

• Solid-core CTs are the usual choice in a new board, where the conductor can be threaded through before it is terminated.
• Split-core CTs clamp around a conductor that is already live and in place, which makes them the retrofit option. The AGL200N-CT uses a split-core CT in ranges of 5, 80, 120, 200 and 300 A.
• Keep the CT and the meter as a pair. The AGL200N-CT’s CT has a milliamp output and is matched to the meter; a standard 5 A secondary CT is not a substitute. Total accuracy always depends on the meter and the CT together.

2. Single phase or three phase?

Meter the supply as it is. A three-phase board needs a three-phase meter; a single-phase meter on one leg reads only that leg, and with unbalanced loads that is a misleading number rather than a small error. A three-phase meter such as the AGW300-WF/C also reports what a single-phase meter cannot: voltage and current unbalance between the phases, and harmonic content (THDv and harmonics up to the 31st), which is useful for finding why a board or a transformer runs warm.

Single-phase meters are the right tool for individual tenants, apartments and final circuits, and for anything fed from one phase by design.

3. Accuracy class

Two IEC standards cover most of what is bought for sub-metering. IEC 62053-21 covers Class 1 and Class 2 meters; IEC 62053-22 covers Class 0.5S and tighter. In practice:

Use	Typical class	Example
Tenant allocation, final circuits, general sub-metering	Class 1	AGL200 (Class 1, IEC 62053-21)
Main incomer, plant, large feeders, energy audits	Class 0.5S	AGW300-WF/C (Class 0.5S, IEC 62053-22)

If the readings are used to bill someone, check what the contract or the local rules require. “MID” is the European regime for meters used in billing; the AGL200 and AGL200N-CT are listed by their manufacturer as MID (Class B), a statement we confirm against the certificate before order.

4. How will the readings leave the meter?

Route	Best when	Watch for
RS-485 Modbus RTU	A BMS, PLC or data logger already exists, or many meters share one cable pair.	Unique address per meter, matching baud rate and parity. See our Modbus RTU vs BACnet MS/TP guide.
Pulse output	A controller or counter with no serial port.	Pulse weight: the AGL200 gives 1000 pulses per kWh, the AGW300-WF/C 6400.
Wi-Fi	No BMS on site, several small sites, or remote reading by browser is the point.	Signal inside a closed metal board; the AGW300-WF/C has an external antenna for this reason. A site network that blocks new devices.

Wi-Fi does not replace the serial port: the AGW300-WF/C keeps its RS-485 Modbus RTU output, so the meter can be read locally whether or not a cloud service is used. For a single-page overview of the web-based service that goes with the Wi-Fi meter, ask us for the Energy Cloud flyer. Service terms come with the quotation.

A special case: PV export limiting and zero feed-in

Many small solar plants must not push more than an agreed power into the grid, or none at all. The usual arrangement is a bidirectional meter at the point of connection that tells the inverter or an energy-management controller how much power is flowing in each direction. The meter only measures; the inverter or controller reduces PV output when export approaches the limit.

• Placement. Put the meter where it sees the net flow between the site and the grid — on the supply side of the loads and of the inverter’s connection — not on the PV branch, which would only show what the panels produce.
• Speed. The AGL200N-CT’s voltage, current and power readings respond within 100 ms, which suits control loops that must react to a cloud passing or a large load switching off. Energy registers update more slowly (within 1 s).
• Direction. Direction depends on the CT orientation. Check import and export with a known load at commissioning.
• The inverter has to read the meter. Before ordering, confirm that your inverter or controller can read this meter over Modbus RTU, and send us its model so we can check the register map.
• Three-phase supplies. A single-phase meter sees one phase. Grid operators differ on whether export is netted across phases or limited per phase, so confirm the rule that applies before choosing one meter or three.

Quick selector

Situation	Meter
One single-phase circuit up to 80 A, read over Modbus RTU	AGL200
Single phase with a split-core CT, bidirectional, PV or storage	AGL200N-CT
Three-phase board read remotely over Wi-Fi, with harmonics	AGW300-WF/C
Three-phase panel analyser with BACnet MS/TP	AG-EMT-300
Compact Class 1 DIN-rail meter, single circuit or three-phase four-wire	AG-SCM-400

A short checklist for the specifier

• Measure the highest current the circuit will carry; choose direct or CT from that.
• Count the phases and decide whether you need per-phase data, unbalance or harmonics.
• Pick the class from the use: allocation or audit.
• Decide how the data leaves the meter before choosing the model.
• For export limiting, confirm the inverter or controller can read the meter, and where the grid operator wants it fitted.

For where to put meters in a building and how to wire the bus, read the sub-metering guide. The full range is on the energy meters page. This guide is also available in German.