Lower speeds/flow rates = lower energy costs.
It is pretty much that simple but of course the downside is that skimming and cleaners don't work as well on lower speeds.
Most of the benefit will be seen with a two speed pump on low speed. Variable speeds can lower the electrical costs a little more but costs a lot more up front.
One other thing that you can easily do is reduce the pump run time. Most pool owners run their pumps much longer than they actually need to.
But I can't tell you for sure what you will pay without a lot more detail on your setup. But to start with:
Current Pump Model
Current Pump Run Time (hrs/day)
Current Filter Pressure (PSI)
Filter Type/Size
Pool Size (gallons)
Plumbing Diameter (e.g. 1.5", 2", 2.5")
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