Building technology energy management system and error diagnosis method

The system (10) has a data processing device (12) comprising a determining unit for determining difference consumption for actual total energy consumption during operation of the system based on prognosticated total energy consumption and measurement values (29) of a measurement device (28). The device comprises a checking unit for checking malfunction in electrical consumers (18-24) and communication channels (32-42) by checking presence of deviated energy consumption criterion. The device comprises an output unit for outputting warning at an output device (14) in case of the malfunction. The data processing device is designed as an industrial personal computer (PC). An independent claim is also included for a method for tracing fault in a building energy management system.