Windmill power is coming to Reston.

A cluster of 55 townhouses, called the Wharf and scheduled to be built next year in the Hunters Woods section of the new town in Fairfax County, will use water pumped by a windmill as the main element of its energy system. The energy generated is expected to cut household power costs by half.

The town houses - to be priced from $70,000 to more than $100,000 - will be clustered around a 2 1/2-acre pond created from water pumped from the ground. Year-round circulating water from the ponds will provide heating and cooling energy for the houses.

When it's too cold or too hot to provide all the energy needed for heating or air conditioning the houses, supplemental electrical energy will have to be used.

Under the Wharf's "solargy" system, energy in te form of heat will be extracted from the water by a device called a heat pump. Heat pumps are already in wide use in new residential and commercial construction in the Washington area, but they convert heat from the air, not water.

Wharf architect Michael L. Oxman of Reston said his project's heat pumps will be able to work more frequently during the winter because heat can be extracted from water more efficiently than it can from the air.

According to Wescorp Inc. of Andover, Mass., which has about 400 so largy systems in operation in the East and Middle West, energy can be extracted from water when the temperature is just above freezing. Conventional heat pumps can work only when the air temperature is above 40 degrees.

Wescorp president Carl Orio said the Wharf will be the largest residential project in the country using his firm's solargy system. Most of the other units in operation are in individual houses, not entire clusters, he said.

"The plan is exciting and very innovative in the way it approaches the problem of energy and its soaring cost," said Supervisor Martha V. Penino (D-Centreville), whose district includes Reston.

Preliminary reviews of the Wharf project by country staff members have called it "an outstanding proposal" that is "truly imaginative and sophisticated."

In addition to reducing heating and air conditioning costs, the project will provide, free of charge, hot water as a byproduct of the solargy process, Wescorp officials said. The average annual energy cost of providing hot water conventionally runs about $240, according to the Potomac Electric Power Co.

Besides pumping water into the pond, the windmill Oxman said, will be an important part of the Wharf clusters design. Most of the open space, instead of being grass that has to be maintained and cut, will be weater, some of it flowing over small waterfalls between the houses.

"The windmill will be in the middle of the pond," Ixman said. "The residents will be able to swim or ice skate to it or take a boat there."

The Wharf, he said, is part of an increasingly popular trend in architecture called "waterscape design." Waterscape design, Oxman said, has been used primarily in the West and Florida as an esthetic feature. But now, he said, with the high cost of energy and possible future shortages, it can be used in a practical way to reduce power needs.