classical thermodynamics do not appear to place a fundamental bound on the power output associated with reversible (Carnot efficient) operation. Most textbooks say that reversibility requires “small” power output, but rarely define what “small” means. The central objective of Ref. \[\] was to find the meaning of “small”, and find a fundamental upper bound on the efficiency of an irreversible system in which the power output was [*not*]{} small.

Ref. \[\] did this for the class of quantum thermoelectrics that are well modelled by a scattering theory, which enables one to straightforwardly treat quantum and thermodynamic effects on an equal footing. It summarized two principal results absent from classical thermodynamics. Firstly, there is a quantum bound (qb) on the power output, and no quantum system can exceed this bound (open circles in Fig. \[Fig:summary\]). Secondly, there is a upper bound on the efficiency at any given power output less than this bound (thick black curves in Fig. \[Fig:summary\]). The efficiency at given power output can only reach Carnot efficiency when the power output is very small compared to the quantum bound on power output. The upper bound on efficiency then decays monotonically as one increases the power output towards the 