As renewable energy generation units become more integrated into power systems, electromagnetic transient (EMT) simulations have become essential for analyzing the dynamic behavior of grid-connected renewable energy systems. How-ever, conventional zero-state initialization methods often fail to ensure a physically consistent steady-state operating point, leading to non-physical transients that degrade both simulation efficiency and accuracy. This paper proposes a power-flow-based initialization method for renewable energy generation units, which guarantees the provision of physically consistent initial values for both state and algebraic variables. Starting from a steady-state power-flow solution, the proposed method systematically initializes the electrical, mechanical, and controller states of renewable energy generation units, ensuring consistency between AC/DC-side variables and the steady-state operating point at the beginning of the simulation. The effectiveness of the method is validated through simulations involving doubly-fed induction generator (DFIG), permanent-magnet synchronous generator (PMSG), and photovoltaic (PV) generation units. The results demonstrate that the method effectively suppresses non-physical transients, improving both EMT simulation stability and computational efficiency.