As such, nuclear power plants are important contributors to grid restoration in the event of blackouts. In particular, NPPs require more time to start up and cannot vary the cycle length much. Also, in many OECD countries flexible operation of NPPs is limited or not authorised by national safety authorities; NPPs can therefore offer only limited flexibility capabilities to the TSO. The inertia of wind turbines is quite low, while solar PV has no inertia.