Replacing an existing single speed pool pump and motor with a high efficiency variable speed pool pump and motor has the potential for significant savings, especially in warmer climates where there is a longer swimming season.
SC Edison estimates for a 20,000 gallon pool that needs cycling one time per day, the average homeowner will save approximately $430 per year using a variable speed, high efficiency pool pump motor over a single speed pool pump motor. This allows for payback on the investment within a few years and may also reduce noise when the motor is running at a lower speed. Energy Savers references a Florida study of 120 home pools which indicated a 40% savings just from the installation of a high efficiency pool pump and motor.
Both sources warn of the importance of properly sizing the new pool pump and motor in order to avoid consuming more energy than necessary for your pool's needs.