Energy-Efficient HPC Green Computing is a computing discipline focusing on maximizing performance-per-watt through hardware design, software optimization, and system management reducing environmental impact — Energy efficiency in HPC addresses growing power costs, environmental concerns, and physical constraints of cooling exascale systems. Hardware Design implements specialized processors optimized for energy efficiency, reduces unnecessary data movement minimizing dominant power consumer, and employs low-power circuit techniques. Voltage Scaling reduces supply voltages decreasing power quadratically, exploits application tolerance for approximate computation enabling aggressive scaling. Power Gating disables idle components eliminating leakage current, balances benefits against wake-up overhead. Efficient Interconnects employs high-radix networks reducing hop counts and average message distances, reduces total power for communication. Memory Systems minimizes memory traffic through better algorithms and data locality, employs efficient memory technologies including 3D-stacked memory. Parallel Algorithms redesign algorithms reducing total operations and communication, may sacrifice sequential efficiency for better parallel efficiency. Power Measurement instruments systems measuring power across components, identifies energy hotspots guiding optimization efforts. Energy-Efficient HPC Green Computing enables sustainable high-performance computing infrastructure.