Sub-metering with Modbus Energy Meters

Why sub-metering?

A building’s main energy meter measures total consumption. Sub-metering divides this into tenant, zone, or system level: floor by floor in offices, apartment by apartment in blocks, HVAC vs lighting vs IT in commercial buildings.

EU Directive 2012/27/EU (Energy Efficiency Directive) requires individual metering for new multi-apartment buildings wherever technically feasible and cost-effective. KNX reads sub-meter data via Modbus and displays it on room controllers, energy dashboards, and sends it to billing systems.

MID certification

MID (Measuring Instruments Directive 2014/32/EU) certifies energy meters for legal billing purposes. A MID-certified meter can be used for:

• Tenant billing in apartment blocks
• Electricity supplier settlement
• Energy performance contracts

Non-MID meters are perfectly fine for internal monitoring and building management but cannot be used for billing. Schneider iEM3155 and Eastron SDM630MCT are popular MID-certified 3-phase Modbus meters widely used across EU projects.

Wiring: direct vs CT

Two wiring methods exist, chosen by the current level of the circuit being measured:

Modbus register map basics

Most Modbus energy meters use a similar register structure based on the Eastron/Carlo Gavazzi convention. Registers are read via Modbus function code 04 (read input registers). Each register is 2 bytes; 32-bit float values span 2 consecutive registers (big-endian).

Register  Parameter              Format    Unit
─────────────────────────────────────────────────
40001     Voltage L1             Float32   V
40003     Voltage L2             Float32   V
40005     Voltage L3             Float32   V
40007     Current L1             Float32   A
40009     Current L2             Float32   A
40011     Current L3             Float32   A
40013     Active power L1        Float32   W
40015     Active power L2        Float32   W
40017     Active power L3        Float32   W
40031     Power factor (total)   Float32   —
40073     Active energy total    Float32   kWh

Read: Function code 04 (read input registers)
Each 32-bit float = 2 consecutive 16-bit registers

A KNX-Modbus gateway (MDT SCN-IP100.03, Loytec LIP-ME201) reads these registers on a configurable polling interval (typically 10–30 seconds) and maps them to KNX DPT 14.xxx group objects, making energy data available on the KNX bus for room controllers, visualisations, and logic modules.

Panel wiring practice

In a DIN-rail panel, energy meters mount below the MCB they monitor. CT-type meters have CTs installed on conductors before (main) or after (sub) the MCB. Key RS-485 Modbus wiring rules:

Example: apartment block sub-metering

A 12-unit apartment block with a complete Modbus sub-metering system:

Grid supply
    │
    ├── Main meter (Modbus addr 1)
    │   Schneider iEM3255, MID, CT 250A 3-phase
    │   → Billing settlement with utility
    │
    ├── Apt 01 meter (Modbus addr 2)
    │   Eastron SDM120, MID, direct 63A 1-phase
    │   → Tenant billing
    │   ...
    ├── Apt 12 meter (Modbus addr 13)
    │   Eastron SDM120, MID, direct 63A 1-phase
    │
    └── Common areas meter (Modbus addr 14)
        WirenBoard WB-MAP3H, non-MID, 3-phase CT
        → Lift, stairwell lighting, parking
        → Internal building management only

RS-485 bus: addresses 1–14, all meters daisy-chained
WirenBoard WB-7 controller reads all meters → MQTT
KNX gateway subscribes MQTT → KNX group objects
Monthly billing reports generated per apartment

The WirenBoard WB-7 controller reads all 14 meters via Modbus and publishes readings to MQTT. A KNX-MQTT bridge (or Home Assistant with KNX integration) makes the data available on the KNX bus for room controller display and generates monthly billing reports per apartment.