Formula for water pressure, flow and power

I feel like this should be readily available, but I cannot find any information about it through google.

I am trying to find a formula that will tell me the amount of energy necessary to inject water into a system at a given pressure.

Put another way: I have a pipe of a known size and water (pipe a) is flowing through it at a know pressure (pressure x). I want to hook a pipe (pipe b) up to that original pipe a to inject additional water into it. I want to make sure that I am injecting the water into that system at a fixed flow (measured in GPM). I need to find out how much energy (say in horsepower or kPA) I would need to use to inject the water.

or graphically what is the energy necessary to inject a constant stream of water at point O, given pressure x?:

PIPE A (pressure x)
-O--------->->->->----------------->->->->----------
|
^
|P
|I
|P
|E
^
|B
|

I know that if pressure x is low, then I will not need very much energy to inject the water into that pipe, and if it is high, I will require quite a bit of energy.

I however, do not know the specific formula that is used to calculate this energy requirement. Any links, or simply the formula, if known, would be greatly appreciated.

Thanks very much.
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 Recognitions: Science Advisor $$P = p*\dot{V}$$ Where: $$P$$ is the power required $$p$$ is the pressure required $$\dot{V}$$ is the volumetric flow rate Make sure your units are consistent, i.e. use the standard units, m, sec, N or Lbf, ft, s. If you are pumping water and need it quick you can use this too: $$HP = \frac{\tex{psi}*\tex{GPM}}{1714}$$ Where: $$HP$$ is the power in horesepower $$psi$$ is the pumping pressure in psi(g) $$GPM$$ is the flow rate in gallons per minute
 I would like to calculate the power need for fulfilling a pressurized airflow stream. With water it is simple but since air is a compressible gas it gets more difficult, or not? I've seen different alternatives. Please help! regards /k