Methods, and apparatus for determining a characteristic of an inertial contribution to an electrical power grid are described. A change in inertia in the electrical power grid based on one or more inertia measurements performed in a first time period is determined. On the basis of the determined change in inertia and data indicative of an inertia contribution characteristic of one or more energy production facilities and/or one or more energy consuming devices in the electric power grid, a characteristic of an inertial contribution to the electrical power grid is determined for a second time period different to the first time period.