Much like electricity, Christians have the power of God in their lives. A Christian who spends more time with God and lives a righteous life will make himself a ready conduit for God's power and will be more useful to God than a carnal Christian. It is this power that determines the level at which Christ will use us for His work. If you don't have much of the Power of God in your life you may only be useful to him in menial tasks of service (you may be an electric razor battery...important, but not life changing). If you walk with God however, He may bestow his blessing and a greater amount of power upon you in order to use you for mightier works.
When I look at the lives of Moses, Joshua, Elijah, Elisha, Peter, and Paul I am convinced that the Lord used them so mightily because of their faithfulness and righteousness. We are all saved by the Grace of God, but your righteousness after salvation and your close walk with God will determine your effectiveness for Him. Endeavor to be a 500 kilowatt transformer, not a AAA battery.