  Accurate inertia estimates and forecasts are crucial to support the system
operation in future low-inertia power systems. A large literature on inertia
estimation methods is available. This paper aims to provide an overview and
classification of inertia estimation methods. The classification considers the
time horizon the methods are applicable to, i.e., offline post mortem, online
real time and forecasting methods, and the scope of the inertia estimation,
e.g., system-wide, regional, generation, demand, individual resource.
Shortcomings of the existing inertia estimation methods have been identified
and suggestions for future work have been made.
