Billie Jenkins is a member of the duo once known as The Ultimate Power. The other member is her sister Christy.
These sister were destined by prophecy to have great power similar to The Charmed Ones however not as powerful, as The Charmed Ones power is a culmination of a long line of witches, fueled by the spiritual Nexus on which they lived, and enhanced by the sisterly trio.
Billy and Christie, the Ultimate Power, demonstrated their strength by destroying the Noxon Demons, thought to be unvanquishable. The Ultimate Power was lost however in the Ultimate Battle, which saw The Charmed Ones vs The Ultimate Power, resulting ultimately in the death of the Triad and Christy.
Therefore Billie is left and the Ulitmate Power is broken. Billie is still very powerful however with enhanced powers of telekinesis and the rare power of Projection but not more powerful than The Charmed Ones, the most powerful good witches of all time.