@XyFreak It's likely because RFTools Power uses a new API in McJtyLib that uses Longs for power. I'm going to write a quick and dirty driver for said API so that this can work with it.
EDIT: Unfortunately OC seems to be capping returned values to the maximum value of a 32 bit integer. Although Vex said that it internally uses doubles, so it should work. Not sure why it doesn't. I'm going to try casting it to a double and see what happens.
EDIT2: I GOT IT TO WORK!!! Uploading to CurseForge now!