Choosing an energy monitoring system

On retrofit monitoring projects, three problems recur. Readings on lightly loaded circuits are several percent out, and nobody trusts them. Cable runs and shutdowns cost more than the meters. Data cannot leave the platform it was bought with. The ten checks below expose these problems before a contract is signed. Ask every supplier the same checks. For each answer, ask for a datasheet figure, a test result or a reference site.

This guide is part of the planning and commissioning learning path. Record the answers in the project worksheet.

Accuracy of the whole measurement

A meter's accuracy class does not include its current transformers. IEC 62053-22 lets a Class 0.5S meter at unity power factor read within ±1.0% between 1% and 5% of rated current, and within ±0.5% from 5% up to maximum current. At a reduced power factor, the limit is ±1.0% between 2% and 10% of rated current, and ±0.6% above that. The CT adds its own error. IEC 61869-2, Table 201, allows a Class 1 CT ±3.0% ratio error and 180 minutes of phase displacement at 5% of rated current. At rated current, the limits are ±1.0% and 60 minutes. Table 202 gives the tighter S classes: a Class 0.5S CT is allowed ±0.75% and 45 minutes at 5% of rated current.

These figures are the limits in the standards. A real CT is usually better, but its error at a given current and burden is mostly systematic, so it does not average out over a month. A calibration of the CT together with its meter can measure and correct it. When the CT is bought separately and paired on site, nobody corrects it. The scope note in the 2003 edition of IEC 62053-22 says that only Class 0.2S and 0.5S CTs have the accuracy these meters need.

Ask for the combined accuracy of the meter and CTs at the lowest normal load of each circuit. Check how the CTs were sized. A common failure is a CT sized to the breaker rating on a circuit that normally draws a small fraction of it. The CT ratio and burden calculator and the CT selection guide cover sizing. The high-accuracy metering guide explains what a class does and does not approve. Billing use needs a legally approved instrument, whatever its class.

Network

The network follows the cable routes and the reporting interval.

Modbus RTU over RS-485 suits meters that are already on a bus, or meters in one switchroom. One segment carries 32 unit loads without a repeater. Every meter needs a unique address, the same baud rate and a cable drop. The bus needs termination at both ends. The RS-485 and Modbus guide gives the limits.

A Zigbee mesh (IEEE 802.15.4 at 2.4 GHz, 250 kbit/s) suits retrofits with meters in several boards and no spare containment. A 1-minute reading is a few dozen bytes, which is a small load for the link. Its range depends on the walls, the metal and the positions of the mains-powered routers. Survey the radio at the hardest location, such as a plant room in a basement or a board inside a steel enclosure.

LoRaWAN suits a small number of points far apart, such as a water meter at a remote building or temperatures across a campus. It does not suit 1-minute electrical data. In the EU868 band, the three default uplink channels have a 1% duty cycle. At spreading factor 12, a 12-byte payload takes about 1.5 s on air. A device that uses only those channels can then send about one uplink every 2.5 minutes.

The site IP network suits meters with an Ethernet port, if IT will provide ports, a VLAN and firewall rules. Every later change then goes through the IT change process. The wired vs wireless guide compares the options in more detail.

Installation and commissioning

For each point, count the panel shutdowns, the metres of cable and containment, and the changes to the panel. A shutdown on a 24-hour production line may have to wait for a maintenance window weeks away. Check whether the meter fits inside the panel. If it does not, check its ingress protection rating for mounting outside the panel. Check whether a qualified electrician can install it without a vendor engineer on site.

Commissioning is the work from installation to a reading that has been checked. On a wired bus, it includes the address, baud rate, parity, termination, register map and scaling of every meter, and the CT ratio set in each meter. Ask the supplier for the time per point and for the records they hand over. The most common commissioning faults are a CT that faces the wrong way, a CT paired with the voltage of another phase, and a wrong CT ratio in the meter. The commissioning checklist shows how to find each fault from the readings.

Cost per point

Compare suppliers on cost per point over the life of the system. Do not compare meter prices. Give each supplier the same cost lines: meter, CTs, cable and containment, installation hours, shutdowns, commissioning hours, software licences (per device, per data point or per site), connectivity, support and the cost to add a point later.

The energy monitoring payback calculator compares the total cost with the expected saving.

Interval and time

The reporting interval sets what the data can be used for. GB electricity settlement uses half-hour periods. Under Regulation (EU) 2017/2195, EU balancing markets use a 15-minute imbalance settlement period, with derogations possible. Demand response and frequency services can need data in seconds or less. Record at 1 minute or faster. You can sum 1-minute data to any longer period, but you cannot divide 30-minute data into shorter ones.

Ask how each value is timestamped. The timestamp must come from the device or gateway at the time of measurement, not from the platform at the time of arrival. Ask how the clocks are synchronised. Ask whether an interval is labelled by its start or its end. A one-interval shift is a common reason why submeter totals do not reconcile with the main meter. The source time and arrival time guide explains the difference.

Access to the data

Ask for the raw interval data, not only reports and charts. Ask which protocols carry it (MQTT, HTTPS, file export, Modbus or BACnet), and in which formats. Check that every value carries its timestamp, its unit and a quality flag. Read the contract for three points: export charges per point or per data volume, a right to a full export of history when the contract ends, and any clause that lets the supplier restrict access to the data.

Security

Ask for a specific answer for each link. On the radio, Zigbee 3.0 encrypts network traffic with AES-128. Ask how devices join the network: a per-device install code is stronger than the publicly known default link key. On the gateway, credentials must be unique to each unit, with no shared default password. Ask how they are rotated. Every connection that leaves the site needs TLS 1.2 or later with certificate validation. Modbus TCP and BACnet/IP have no built-in encryption, so keep them on a separate network segment.

Ask whether the system continues to operate, and store data, when the supplier's cloud service is not available.

Firmware updates

Under the EU Cyber Resilience Act, which applies from 11 December 2027, the manufacturer must declare a support period for security updates. The support period is at least five years unless the product's expected use time is shorter. A panel meter is often left in service for longer than five years. Ask for the support period in writing, and ask what happens to devices that are still installed when it ends.

Ask how firmware reaches an installed device. An update that needs a site visit to each panel will usually not be installed. Ask whether images are signed, whether a device returns to its previous firmware if an update is interrupted, and whether an update can go to one device before the rest of the fleet. The over-the-air updates guide explains the methods.

Behaviour during failures

Three events will occur on every site: a device goes offline, the gateway restarts, and the internet link fails. In each case, the correct result is a gap that is marked as missing. A zero is wrong, because it tells the reader that the load stopped. A repeated last value is also wrong, because it tells the reader that nothing changed.

When the link returns, stored data must be sent with its original timestamps and must fill the gap. Ask how long the device and the gateway can store data. Ask whether data held in memory can be lost at a power cut. Replay can deliver a value twice, so the receiving system must accept a second copy of the same point and timestamp without counting it twice. The stale, missing and invalid data guide and the store-and-forward sizing guide cover both points.

Scaling to many sites

A multi-site programme usually starts with a pilot. Regulation, such as the EU Energy Efficiency Directive or ESOS, often sets its schedule. At twenty sites, the most important scale question is whether configuration can be copied. Device definitions, point names, units, alarm thresholds and export settings must go from one site to the next as a template. If each site is configured by hand, point names drift, and portfolio reports stop matching across sites. Ask also how gateways are monitored and updated remotely. Ask what else the same system can measure: gas and water meters with pulse outputs, heat meters, temperature, and existing Modbus meters.

Pilot acceptance

Run the pilot at the hardest representative site, and write the pass criteria before it starts. Here is an example set:

1. Interval energy agrees with an independent reference meter within the combined error budget of the two chains. Calculate that budget as in the accuracy example. Do not use a round number. Run the comparison for at least 14 days, including a weekend and the lowest normal load.
2. After replay, each device delivers an agreed share of its expected intervals, for example 99.5%.
3. A 4-hour internet outage, a gateway power cycle and one device powered down each give a marked gap. The first two gaps must later fill with original timestamps.
4. Each point passes the identity, sign and phase checks in the commissioning checklist.

Roll out the system that passes these tests.

Where EpiSensor fits

A ZEM electricity monitor ships with its current sensors connected. EpiSensor calibrates each ZEM with the sensors it ships with, and states Class 0.5S to IEC 62053-22 for the meter and sensors together. That removes CT pairing, ratio setting and burden checks from the site. The ZEM is IP67 and double-insulated, and operates from −30 °C to 60 °C, so it can mount outside a panel. It reports over the Zigbee mesh with AES-128 encryption and stores up to 70,000 readings in flash.

The ZGW-20 Gateway runs Edge, which keeps local history and needs no EpiSensor cloud service. When a supported destination fails, Edge queues data on disk and replays it with the original timestamps. Edge sends data over MQTTS or HTTPS, or as a scheduled file export to FTPS or SFTP, in JSON or CSV. Edge also serves data to a BMS as a Modbus TCP server, and that link belongs on the BMS network. Edge updates Zigbee device firmware over the air. Edge itself updates through its snap channel or the desktop app. The same Gateway also reads pulse counters, temperature sensors, 4 to 20 mA inputs, existing Modbus meters and LoRaWAN devices.