A special case, due to its intrinsic importance as well as due to the particular challenges that it poses, is constituted by power generation, which is discussed in the section below. For instance, dispatchable capacity needs to be adequate at all time in order to ensure that sufficient active power is available in the system to meet demand, otherwise the frequency of the system would deviate from the desired 50Hz or 60Hz.21 Lack of sufficient active power capacity may also lead to “brownouts”, which is a reduction of the system voltage. This usually only occurs in regions with weak or isolated grids, where the lack of active power leads to reduced frequency, which in turn may lead to reduced generator voltages. To keep the voltage of an electric system at appropriate levels, sufficient generation of so-called reactive power is also necessary.