Si8519 AC Current Sensor usage
I only looked at the datasheet quickly, but the main output is analog.  That means you have to run it into a A/D input.

However, it looks like you have to get envolved in resetting the internal integrator.  I didn't really read that section, so maybe there is a way to have the chip reset itself automatically from the incoming power line cycles.  It also mentioned something about the output being pegged during the negative half of the cycle, so it looks like you have to be aware of when the line half-cycles are.  This kind of real-time control is probably easier to do with a microcontroller, then send nicely cooked digital data to the RPi.

This part looks tricky, so you have to read the datasheet very carefully before planning out your strategy.