Restoring power after a wide-area power outage can be difficult, as power stations need to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.
To provide a black start, some power stations are typically equipped with small diesel generatorss which can be used to start larger generators (of several megawatts capacity), which in turn can be used to start the main power station generators.
To do: Black start after nuclear war scenarios