For technical reasons, to ensure frequency and voltage stability, the amount of electricity fed into the grid must at all times be exactly the same as the amount that is withdrawn by consumers. Producers and consumers are grouped into what is known as balancing groups. Each balancing group is obliged to balance energy consumption and generation within the group. In the event of unforeseen fluctuations in generation (e.g. due to power plant failures or deviations in wind speed forecasts) or in consumption levels, the control area operator must ensure the balance of energy in the grid by activating or deactivating special backup power plants. The costs of these balancing measures are charged to the balancing group that was responsible for the fluctuation. In Austria, such fluctuations are calculated and invoiced through an independent balancing group coordinator; APCS Power Clearing and Settlement AG performs this function for the APG control area.