An apparatus (10) for measurement of power in an electric power transmission line (20) is disclosed. The apparatus (10) comprises a magnetic field sensor (30) for measuring the magnetic field (35) at the electric power transmission line (20) and transmitting magnetic field data (37) to a processor (40). The magnetic field sensor (30) is arranged at a minimum threshold distance from the electric power transmission line (20). The apparatus (10) also has a voltage sensor (50) arranged distally from the magnetic field sensor (30) for transmitting voltage waveform data (55) to the processor (40) and a transfer function calculator (60) for calculating the relationship between the transmitted voltage waveform data at the voltage sensor (50) and transmission line voltage waveform data (55).