Continuing the discussion from Edimax SP-2101W true RMS or not?:
As posted in the thread above by @kobuki linked to a rather interesting teardown and hack project of a Belkin WeMo smart plug which also does energy monitoring. There seem to be some points of interest to learn from this post. I plan to have a more detailed read next week:
Project | SmartEE the Smart Plug | Hackaday.io
Belkin seems to use the 78M6610 energy monitoring chip that reports via I2C, SPI or UART.