Looking into this project currently, i.e. use of a AC current sensor + pi/HA for monitoring energy usage. Currently trying to understand how to read from these sensors with a pi, found these useful links which indicate that a custom circuit is required.
http://www.homautomation.org/2013/09/17/current-monitoring-with-non-invasive-sensor-and-arduino/
May purchase one of these: https://shop.openenergymonitor.com/emonpi-3/