It depends on the particular membranes that are employed. Example, if conversion rate is greater than 60 percent, I believe the savings will be small. However, if the membranes conversion rate is say only 30 percent, the savings can be large. Here is why:
A million gallons per day plant uses membranes with a 30 percent flux rate, than to produce the 1,000,000 gallons you will have to pump
Total water = 1,000,000/.30 = 3,333,333 gallons of water at RO pressure (Say 1,000 psi). Required horsepower is: HP = (3,333,333 * 1,000)/1714 = 1944.
However, the pure water is only 1,000,000 gallons, so the HP = (1,000,000 * 1,000)/1714 = 583. This is a savings of the cost of 1,361 more horsepower over the pumping schedule (normally 8 hours). In a year time, the horsepower savings is (365 days * 1,361 hp) = 496,765 yearly hp savings.