This paper presents a highly power-efficient instrumentation amplifier. It adopts an inverter stacking amplifier (ISA) based 1 ^st -stage that realizes 4× current reuse, thereby greatly reducing the supply current. To boost the power efficiency and enable its robust operation under 0.6V supply, the tail current sources are removed. A high CMRR of 84dB is maintained by combining chopping, closed-loop biasing, and inherent high impedance degeneration. A 3-stage topology with a class-AB last-stage realizes high loop gain and power-efficient dominant-pole compensation. A prototype tail-less ISA in 180nm achieves 1.38uV rms input referred noise (IRN) within 8-kHz BW, while consuming only 2.7uW. This leads to a power efficiency factor (PEF) of 0.96. To authors' best knowledge, it is the best reported PEF to date.