There are a variety of whole-home water purification systems on the market.
GE and others offer filtration and softening systems. Many are salt-based for softening with an added RO (reverse osmosis) for drinking purification. The RO is usually in one or two locations, as a separate system is required. There is one non-salt based system I know of made by EWS, Inc.
Related to energy, all these systems are efficient as they use only a small amount of electricity to cycle the system. I imagine your main energy consumer is the pump that draws water from the well.
In this case, you'll have to research what pumps are available for your requirements. Review the power consumption specifications on the pumps.
There also is a separate device that can improve electrical efficiency on all electric motors in your home. It's a simple capacitor that is made by many manufacturers. You can find more details about the technology at MoneyMatters101 and CleanAir365. You'd have to search for companies in your area that have them. Remember to search for home units, as they're made for commercial and industrial applications as well.
Best of luck!