Methods, computer software and apparatus for use in an electric power grid are described. Measurement data is received intermittently or continuously from a plurality of measurement units, indicating values of at least one measured characteristic at respective measurement locations. The at least one is characteristic is monitored, and it is determined, based on the monitoring, that there has been a change in electric power flow in the electric power grid. Responsive to the determination, component data is accessed relating to characteristics of components of the electric power grid involved in provision and/or consumption of electric power. Based on received measurement data relating to the change and the accessed component data, a power flow characteristic of the grid in a given time period is determined.