I know this all can be a bit confusing until you actually work with the parts in your hand so let me try to clear up a bit:
If you currently have a 846 , 4u , 24bay CHASSIS WITH a PDB and you have 1 or 2 power supplies - then all you need is a back-plane (ie: BPN-SAS1-846EL1 is a back-plane and expander).
Attached is a picture ive marked of a PDB (BLUE circle) and a PowerSupply (green circle , click thumb to enlarge).
The power supply slides INTO the chasis and connects/plugs into the PDB (an 846 4u chassis can accept one or two power supplies, but only has ONE PDB)
PDB= Power Distribution Board
Let me know if you have any ?