  In an article by Garc\'ia-Pintos et al. [Rev. Lett. 125, 040601 (2020)] the
connection between the charging power of a quantum battery and the fluctuations
of a "free energy operator" whose expectation value characterizes the maximum
extractable work of the battery is studied. The result of the closed-system
analysis shows that for a general charging process the battery will have a
nonzero charging power if and only if the state of the battery is not an
eigenstate of the free energy operator. In this Comment, we point out a few
mistakes in the analysis and obtain the correct bound on the charging power.
Consequently, the result for closed-system dynamics is in general not correct.
