Pump sizing has absoultely NOTHING to do with HP.
Does this make sense.... "I need a car that will do 25 MPH - what HP do I need?"
It's like comparing a riding lawnmower to a moped. The mower is probably 12HP - can it do 25mph? Now what about a little motorized scooter or moped - much less HP, faster speed. [I don't even know what Hp those motors are - probably 3/4 ??]
Hopefully you see my point.
1. Determine max flow needed/wanted.
2. This will give you any friction head.
3. Determine elevation head.
4. Determine the 'pressure' head (i want x psi at this much elevation.
You now have your DUTY POINT. Look at pump curves to match that duty point. Then you will know how much HP that specific pump end needs.
I've found that this link is an excellent source for learning how to size a pump. Just download the
6th section:
http://www.us.grundfos.com/web/download.nsf/DownloadViewOne/8A92FF77621B7EC588256F7F006185B5?OpenDocument
The people who do this for a living don't really need to "Do the math" - they know what a certain HP can do (only after you give them A- the TDH or B- the flow)