Why does the power register return an impossibly large value?

This is almost always a byte-order mismatch. Register 252 on the Kostal inverter uses little-endian word order (CDAB, also called float32s or swapped float). If the inverter Modbus or SunSpec byte order is set to big-endian (ABCD), the float32 bytes are read in the wrong sequence, producing garbage values in the trillions of watts. Set byte order to little-endian (CDAB) in the inverter web interface under the Modbus or SunSpec settings.

The register reads what looks like house consumption, not grid power. Why?

Register 252 changes meaning depending on which sensor position the KSEM occupies in the inverter configuration. For grid import and export, the KSEM must be assigned to sensor position 2 (grid connection) in the inverter settings. If it is assigned to sensor position 1 (home consumption), the register reports house consumption with the opposite sign convention.

The gateway connects but the power value never updates. What now?

Confirm that Modbus TCP is actively enabled on the inverter, not just the KSEM. The inverter's Modbus TCP server listens on port 1502 by default, not the standard port 502. Verify port 1502 is reachable from the ModbusCloud Gateway, that no firewall rule blocks it, and that the unit ID is set to 71 in the portal template assignment.

Values oscillate between correct readings and near-zero every few seconds. What causes this?

Intermittent oscillation typically means another Modbus client is competing for the same TCP connection to the inverter. The Kostal inverter allows only a limited number of simultaneous Modbus TCP connections. Ensure no other home-automation system (such as a parallel EVCC or ioBroker instance) is polling the same inverter address and port at the same time.

Free Modbus template: Kostal Smart Energy Meter (via inverter) (grid)

Name: Kostal Smart Energy Meter (via inverter) (grid)
Brand: Kostal
SKU: 16a5ede6-4cd7-405f-9302-c1211cd67cbc

The Kostal Smart Energy Meter (KSEM) is a three-phase energy meter used in Kostal solar installations to measure grid import and export at the grid connection point. This template reads the KSEM's grid power value via the Kostal inverter's own Modbus TCP interface, using register 252 to expose total active power in watts. Positive values indicate power drawn from the grid; negative values indicate feed-in. Connecting the ModbusCloud Gateway to the inverter's LAN port gives installers and building owners continuous visibility into real-time grid exchange without a separate meter connection.