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How the HEMS works: load management and PV charging

5 min read · Updated 06/2026

At runtime, the HEMS performs two technically separate jobs: load management to protect the fuse, and PV charging to make use of solar surplus. Both functions can be active independently. A prerequisite is an activated HEMS on the CEM wallbox and, depending on the mode, a supported energy meter.

The two functions at a glance

Load management ensures that the sum of charging currents stays below the fuse limit — with or without accounting for other household loads. PV charging additionally routes any available PV surplus to the vehicle. The two jobs run independently: an installation can be operated purely for load management, whereas PV charging in turn requires a configured house energy meter.

Static load management

Without a configured meter, the HEMS distributes a fixed current budget evenly across all paired wallboxes. Other loads in the building are unknown, so the limit has to be chosen conservatively — in the worst case, full household consumption and charging run in parallel.

Diagram of static load management: 32 A fuse limit split into max. 10 A per wallbox.

Dynamic load management

With a house meter, the HEMS observes household consumption and recalculates the charging budget continuously: available = grid limit − house load. Charging power grows and shrinks accordingly, without putting the fuse at risk.

Diagram of dynamic load management: 63 A grid connection minus 20 A house load leaves a 43 A charging budget for the wallboxes.

PV charging

The HEMS calculates the PV surplus (generation − house load) and routes it to the vehicle instead of exporting it to the grid. Prerequisites: a configured house meter, PV mode enabled in the wallbox (see "Configuring PV optimization"), and currently exactly one wallbox — multiple are not supported.

Diagram of PV charging: 7.0 kW PV generation minus 2.8 kW house load leaves a 4.2 kW surplus routed to the wallbox instead of being exported to the grid.

Supported energy meters

For dynamic load management and PV charging, the HEMS needs a supported energy meter at the grid connection point. The meter is connected via Modbus TCP or Modbus RTU through an RTU-to-TCP converter. Currently supported:

  • ABB B21 (Modbus TCP/RTU)
  • ABB B23 (Modbus TCP/RTU)
  • Carlo Gavazzi EM111 (Modbus TCP/RTU)
  • Carlo Gavazzi EM112 (Modbus TCP/RTU)
  • Carlo Gavazzi EM24 (Modbus TCP/RTU)
  • Carlo Gavazzi EM24-DIN_E1 (Modbus TCP only)
  • Carlo Gavazzi EM340 (Modbus TCP/RTU)
  • Eastron SDM120 (Modbus TCP/RTU)
  • Eastron SDM630 (Modbus TCP/RTU)
  • Eastron SDM630 (IT grid, Modbus TCP/RTU)
  • Hager ECR180D (Modbus TCP/RTU)
  • Hager ECR380D (Modbus TCP/RTU)
  • KOSTAL Smart Energy Meter (Modbus TCP/RTU)
  • Negotica P1 Gateway (Modbus TCP/RTU)
  • Schneider iEM3155 (Modbus TCP/RTU)
  • Schneider iEM3555 (Modbus TCP/RTU)
  • Shelly Pro 3 EM (Modbus TCP only)
  • Siemens PAC1600 1×230 V (Modbus TCP/RTU)
  • Siemens PAC2200 (Modbus TCP only)
  • Siemens PAC2200 (IT grid, Modbus TCP only)
  • Steca PV inverter (proprietary connection)
  • Xemex P1MB 1.0 (Modbus TCP/RTU)
  • SunSpec-compliant devices (Modbus TCP/RTU)