Dependable power measurements are the backbone of energy-efficient computing systems. The IBM PowerNV platform offers such power measurements through an embedded PowerPC 405 processor: The On-Chip Controller (OCC). Among other system-control tasks, the OCC provides power measurements for several domains, such as system, CPU, and GPU. This paper provides a detailed description and an in-depth evaluation of these OCC-provided power measurements. For that, we describe the provided interfaces themselves and experimentally verify their overhead (3.6 µs to 10.8 µs per access) and readout rate (24.95 Sa/s). We also study the consistency of the reported sensor readouts across the measurement domains and compare it to externally measured data. Furthermore, we estimate the internal sampling rate (1996 Sa/s) by provoking aliasing errors with artificial workloads, and quantify the errors that such aliasing could introduce in practice (for power consumption of processors 12% in our experimental worst-case scenario). Given these insights, practitioners using the IBM PowerNV platform can assess the quality of the embedded measurements, permitting sought-after energy efficiency improvements.