A single speed pump runs on 3,450 rpm and consumes approx. 2100 W. Using an average price of 0.15 $/kWh, a runtime of 10 hours/day and 5 months/year of swimming season, this pump would cost 3.15 $/day and consume 21 kWh/day or 472.50 $/year and 3,150 kWh/year.
By cutting the speed in half to 1,725 rpm, the power consumption will be 262 W according to the pump affinity law. The flow has been reduced by 50% which in turn will increase the runtime to 20 hours/day. The cost would be 0.79 $/day and consumption would be at 5.25 kWh/day or 118.13 $/year and 787.5 kWh/year. This is an annual saving of $354.37 or 75%. In less than four years you would see a return of your investment (ROI).
The variable speed pump (VSP) efficiency can be greater than 90% compared to a single speed pump. The return of investment (ROI) is approx. three to four years depending on the set-up and design.
Running the pump during Off-Peak hours saves even more money. Water feature pumps can also be switched to VSPs.
Manufacturers like Hayward and Pentair offer Energy Calculators on their websites.