Abstract: In the synchronous generator dominant power system, the mechanical rotational inertia is the dominant factor that determines the frequency dynamics, and is referred to as the system inertia. With the increasing penetration of renewable energy sources, the mechanical rotational inertia keeps declining, its leading role is weakened, and the existing cognition of the inertia of the power system is no longer applicable. The paper first reviews the physical nature of inertia as a measure of the ability to hinder the change of state of an object, and models the power system as an energy supply chain containing various energy forms and components at different scales. The inertia is generalized to mechanical rotational inertia, electrical inertia, thermal inertia and fluid inertia. In energy supply chain, the “inertia pair” composed of storage inertia and channel inertia on the same scale determines the dynamics of the scale. On the whole scale, multiple “inertia pairs” at different scales determine the sequential dynamic characteristics in the time domain and the cascading filtering characteristics in the frequency domain. Then, it is further revealed that the source of inertia demand is energy demand, storage units need to have enough instantaneous energy release to cope with the energy supplement lag caused by the transmission channel, and the entire energy supply chain needs to store enough local energy to support decentralized control. The inertia cognition of the power system from the energy perspective is helpful to evaluate the inertia demand of new power systems and provide theoretical guidance for the method design of active power control of new power systems.