Abstract: With the increase of renewable energy penetration, the inertia of power systems decreases gradually, and the problem of frequency stability becomes increasingly serious. To ensure the safety and stability of power systems' operation, renewable energy units need to provide inertial support. This paper proposes a method to determine the requirements of inertial for renewable energy units by taking the space-time distribution characteristics of frequency into account. First, the importance of the inertial requirements for renewable energy units is discussed from the perspective of frequency stability. Then, the definition and calculation method of the inertia of renewable energy are clarified. Next, the frequency response model of power systems is established, which reflects the space-time distribution characteristics. The method and procedure of determining the inertial requirements of renewable energy units are proposed. Finally, the effectiveness of the proposed method is verified with case studies in an example power system.