A rack computer system can provide data indicating electrical power consumption by separate sets of the mass storage devices, including separate individual mass storage devices, of the rack computer system. A power sensor can be electrically coupled to a power transmission line for each mass storage device. The power sensor can be coupled to the power transmission line externally to the mass storage device. The power sensor can be an internal power sensor of the mass storage device, where a mass storage device microcontroller transmits internally-generated data to an external power monitoring system. A microcontroller can transmit the data to a baseboard management controller via a side-band connection between the mass storage device and the controller. The data can be transmitted via an in-band connection between a baseboard management controller and an instance of firmware which accesses internally-generated data from mass storage device microcontrollers.