A solar-heated model home described by its designer as energy-efficient will be open to the public today and Sunday in the Newbridge community in the Largo area of Prince Goerge's County.

Solar Thermal Systems, a division of Exxon Enterprises Inc., designed the two-story house for the builder, Poretsky & Starr, which is developing the Newbridge property, formerlly called Northampton.

Solar-collecting panels are located on the back of the model house, which has four bedrooms and 2 1/2 baths.

No price has been set on the dwelling, which will be retained by STS for several years of monitoring and research. A spokesman for Poretsky & Starr said that houses with varied solar applications will be built for future buyers at the development.

The solar system in the model house works in tandem with a special heat pump designed for STS to provide back-up interior heating and air conditioning.

When the sun provides enough heat, a hot water storage tank that collects solar energy from a roof collector will provide heat to an exchange unit. Then air passed through the exchange units is heated and circulated through the house.

When solar energy is not available and the water temperature falls below 90 degrees, the system relies on the electric heat pump. The system also heats water for domestic use.

A spokesman for STS also said that the house has been built with thick insulation, an energy-efficient fireplace, double-glazed windows and a heat-retaining brick floor in the family room that has a window placed to admit solar rays in the winter when the sun is in a more southerly position.

The Poretsky & Starr firm says it pioneered in energy-efficient houses here, using features that save enough energy to pay back the cost within seven years.

The Exxon solar house is located at 1404 Golf Course Dr. in Newbridge, which is 2 miles east of Beltway exit 33E on Central Avenue (Rte. 214).