Methods, and apparatus for determining inertia within a synchronous area of an electric power grid are described. A frequency characteristic relating to a frequency of electricity flowing in the electric power grid is measured, a magnitude relating to a power flow modulation is determined based on data relating to power characteristics of one or more power units arranged to consume electric power from and/or provide real and/or reactive electric power to the electric power grid, and a frequency response characteristic associated with at least one area of the electric power grid is determined on the basis of the measured frequency characteristic and the determined magnitude characteristic. This enables frequency response characteristics within a synchronous area of the electric power grid to be easily determined.