Powermon is a small utility that reads the CPU internal power counters, calculates the current power consumption and displays it together with some nice statistics on an interactive curses interface. At this time Powermon is exclusive for the FreeBSD operating system, it's based upon FreeBSDs cpuctl(4) interface.
All Intel CPUs starting with Sandy Bridge are supported. Older Intel CPUs and CPUs from other vendors don't expose the necessary performance counters. Additionally some server CPUs like those based on Haswell and Broadwell do not provide the necessary data.
Powermon needs to know some informations about the CPU. Some of these informations are determined from the CPUID and several MSRs, others are read from internal tables. If a CPU is unknown to Powermon it may not work, therefor the necessary informations can be given at command line.
All Intel CPUs since Sandy Bridge feature a co-processor for power management called Power Management Unit (PMU). The PMU has an accurate view over the current power consumption. Some data about the current power consumption is written into MSRs. Powertop reads this MSRs, applies some correction values and calculates the power consumption in joule.
Additionally the PMU gives the abbility to set power limits, e.g. to tell the CPU or one component (x86 cores, GPU, etc.) not not comsume more power than the given value. This is currently not implemented by Powermon.
More informations can be found here: Intel® Power Governor