A power value for a low-voltage AC circuit, such as low-voltage circuit breaker, is derived. A voltage between conductors of the low-voltage AC circuit is ascertained and a current of at least one conductor is ascertained by a Rogowski coil, which outputs an analog voltage that is an equivalent to the level of the electric current. The analog voltage of the Rogowski coil is integrated and then digitized, and the level of the voltage is digitized. A power value is calculated in each case from a sample value of the current and an interpolated voltage value. The interpolated voltage value is in each case ascertained from two successive sample values of the voltage such that a phase error generated by the Rogowski coil or/and the integrator is compensated for in relation to the electric current in the conductor and a phase error-free power value is ascertained.