The best way to choose a pump is to graph the system curve and prospective pump curves on the same graph.
The best pump choice is where the pump curve crosses the system curve at the desired gpm and the pump curve is at or near the best efficiency point.
Many manufacturers publish pump curves that show where the best efficiency range is.
For example, the link shows a set of pump curves with efficiency ranges .
https://pentairpool.com/~/media/websites/pool/downloads/pumps/performace%20curve%20images/challengerhhseriescurve.jpg
The best efficiency point will usually be near the middle of the best efficiency range.
Some manufacturers will identify the best efficiency point, but you typically have to search for it in the literature.
Most of the time, a variable speed pump will be a good choice.
Pumps use more power as the resistance is decreased because the flow rate increases. Higher flow rates means more power.
The best efficiency point is where the gpm/power is highest.
As long as you stay away from the ends of the pump curve, you're usually good. The efficiency isn't that much different.
When generating a system curve, you should consider how the curve can change due to variables such as a dirty filter etc.
Some variable speed pumps can be set to a specific flow rate to help maintain flow for specific applications, but the pressure limits should be programmed in to prevent overpressure problems.
The main questions that you need to know are what flow rate do you want and what pressure will be generated at that flow rate.
What is the intended application for the pump?
