The energy savings can be significant; it depends on what you have now, and how you use the VS pump.
BUT, VS pumps are far less reliable than standard inground single speed pumps from Hayward, Pentair or Zodiac, primarily due to electronic failure. We suspect that thunderstorm related voltage surges are to blame, since VS problems seem more common on the East Coast, than on the West. But we don't really KNOW that.
If you want energy savings with reliability you may want to look at 2 speed pumps. A carefully selected 2 speed can be nearly as efficient as a VS pump, and will be much more reliable.
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