EN: An energy transformation is efficient if a large fraction of the input energy becomes useful output. - Efficiency = (useful output energy) / (input energy) × 100%. - Real systems are never 100% efficient because some energy becomes unwanted thermal/sound energy due to friction and resistance.

中文：能量转化“高效”表示输入能量中有较大比例变成了有用的输出。 - 效率 = 有用输出能量 / 输入能量 × 100%。 - 真实系统不可能 100% 高效，因为摩擦/电阻等会使部分能量转为不想要的热能/声能。