KNX Peak Load Management: Demand Meter Monitoring and Load Shedding

Demand charge tariff and the cost of a single spike

The demand charge (also called capacity charge, kW charge, or leistungspreis) is levied by network operators on commercial and industrial connections above certain thresholds — typically starting at connection sizes of 25-100 kVA. The charge applies to the highest 15-minute demand recorded in the billing month, and a single uncontrolled load-on event can set the demand for the entire month.

Example: a building with a contracted peak of 50 kW and a demand tariff of 8 EUR/kW/month. Normal peak demand is 38 kW. One afternoon, an EV charger (22 kW), heat pump (8 kW), and electric boiler (12 kW) start simultaneously, pushing demand to 58 kW for 20 minutes. That single event adds 8 EUR/kW times 58 kW = 464 EUR to the month's bill instead of 304 EUR — an increase of 160 EUR from one unmanaged coincidence. KNX peak load management eliminates this with a simple Modbus meter and logic controller.

Eastron SDM630 demand meter: Modbus TCP setup

The Eastron SDM630 is a three-phase DIN rail energy meter with RS485 Modbus RTU output. For KNX network integration, use the SDM630-Modbus-MID variant with an RS485-to-Ethernet converter (such as the Waveshare RS485 to Ethernet converter) to expose the meter on Modbus TCP, or connect directly to an MDT Modbus RTU Gateway on the DIN rail.

SDM630 Modbus RTU: 9600 baud, 8N1, unit ID 1 (default)
All registers: float32, 2 registers per value (FC3 read)

Register 0x0034 (52 dec):  Total system power (W), real-time
Register 0x0036 (54 dec):  Total system VA (apparent power)
Register 0x0156 (342 dec): Import Wh since reset (energy total)
Register 0x0158 (344 dec): 15-minute demand: maximum active power (W) — rolling
Register 0x015A (346 dec): 15-minute demand: current period active power (W)

Poll group 1 (real-time power): reg 0x0034 every 10s
  → KNX GA 5/0/1 DPT 9.001 (W)

Poll group 2 (demand): reg 0x015A every 10s
  → KNX GA 5/0/2 DPT 9.001 (15-min demand W)

Poll group 3 (peak demand): reg 0x0158 every 60s
  → KNX GA 5/0/3 DPT 9.001 (recorded peak W)

Load priority table and shedding sequence

Before configuring KNX logic, define the load priority table: which loads are shed first (highest flexibility, lowest consequence) through to which are essential and never shed. The table must be agreed with the building owner and documented in the KNX project file.

KNX shedding logic: thresholds, hysteresis, and re-enable

The MDT SCN-LCRM.01 logic controller implements the two-stage shedding sequence. A 45 kW threshold (90% of the 50 kW contracted demand) triggers priority 1 shedding. If demand remains above threshold 2 minutes later, priority 2 shedding follows. Re-enable uses a lower threshold (35 kW) with a 5-minute hold to prevent rapid oscillation — loads switching off and on every minute would cause occupant complaints and potential appliance damage.

Input: KNX GA 5/0/2 (15-min rolling demand, W)

Stage 1 shedding — demand > 45000W:
  IF GA 5/0/2 > 45000 for 0s (immediate):
    → Write EV charger current = 0A (see EVBox section below)
    → Write UFH zones 3-8 GA = OFF (KNX actuator 6 channels)
    → Write pool pump GA = OFF
    → Set flag: "stage1_shed = true"

Stage 2 shedding — demand still high after 2 min:
  IF GA 5/0/2 > 45000 AND stage1_shed = true for 120s:
    → Write dishwasher socket GA = OFF
    → Write washing machine socket GA = OFF
    → Write towel rails GA = OFF
    → Set flag: "stage2_shed = true"

Re-enable — demand below lower threshold for 5 min:
  IF GA 5/0/2 < 35000 for 300s:
    → Write UFH zones 3-8 GA = ON (restore)
    → Write pool pump GA = ON
    → Write dishwasher socket GA = ON
    → Write washing machine socket GA = ON
    → Write towel rails GA = ON
    → Write EV charger current = rated value (restore)
    → Clear all shed flags

EV charger shedding: graceful current reduction via Modbus

Switching the EV charger supply circuit off via a KNX relay causes an OCPP session abort — the charger loses power and the OCPP central system receives an unexpected disconnection. Repeated hard shutoffs can corrupt charger state and require manual reset. The correct method is to write the maximum charge current to zero via Modbus or OCPP SetChargingProfile, which pauses charging gracefully while keeping the charger in an active session state.

EVBox BusinessLine Modbus TCP: IP 192.168.1.60, port 502

Write max charge current — register 4128 (unit ID 1):
  Value 0 = pause charging (EV suspends, session active)
  Value 32 = 32A (resume full charging)
  DPT: uint16, in amps

KNX Modbus Gateway write object:
  Write trigger: KNX GA 5/1/1 (DPT 5.010, 0-32A)
  Modbus write: reg 4128, unit 1

Shedding sequence:
  Peak load logic writes GA 5/1/1 = 0 → Modbus writes 0A to reg 4128
  EVBox suspends: charger status = Suspended EV / SuspendedEVSE
  OCPP session remains active (no session abort, no RFID re-auth needed)

Re-enable sequence:
  Logic writes GA 5/1/1 = 32 → Modbus writes 32A to reg 4128
  EVBox resumes: charger status = Charging
  Energy meter GA 5/1/2 (kWh) continues accumulating session energy

KNX peak demand visualisation: MDT Glastaster II gauge

The MDT Glastaster II touchpanel (or Gira X1 visualisation) displays the live 15-minute rolling demand as a bar gauge with a red zone above the shedding threshold. The KNX GA 5/0/2 (DPT 9.001, W) feeds the gauge value object. A separate binary GA (5/0/4, DPT 1.001) signals when stage 1 shedding is active, changing the panel background or activating a warning icon.

The historical peak demand (register 0x0158 from SDM630) is displayed as a separate value widget — useful for verifying that the KNX peak management system is working and the monthly peak is trending below the contracted level. A KNX data logger (ARISTO BewO) records the 15-minute demand time series for monthly reporting to verify demand charge avoidance and provide evidence of system effectiveness to the building owner.