As for powertop on Linux, on FreeBSD you have powermon (from Ports):

# kldload cpuctl # powermon

                  Intel(R) Core(TM) i7-2640M CPU @ 2.80GHz
                      (Arch: Sandy Bridge, Limit: 44W)



   4.98W [=======>                                                           ]



 Package:           Uncore:             x86 Cores:          GPU:
 Current: 4.98W     Current: 3.34W      Current: 1.44W      Current: 0.21W
 Total: 14.37J      Total: 9.79J        Total: 3.62J        Total: 0.96J

Also for power management, powerdxx (from Ports) is better then powerd:

/etc/rc.conf powerdxx_enable=YES powerdxx_flags="-n hiadaptive -a hiadaptive -b hiadaptive -m 1600 -M 3000"