WASHINGTON — As the last truck pulls away and the hardhats come off, 20 solar-powered houses built by university students and associates from around the world will open to the public Friday on the National Mall.

The Department of Energy challenged students in the third biennial Solar Decathlon to build their own 800-square-foot homes that offer the style and comfort consumers want and the energy efficiency the world needs.

Through Oct. 20, more than 100,000 people are expected to visit the houses that students from the USA, Canada, Germany and Spain have designed, constructed, transported and rebuilt for the competition. The teams were selected by experts from the DOE and its National Renewable Energy Laboratory.

The houses "have to be aesthetically pleasing, architecturally stunning, and it has to work," decathlon director Richard King says. "And we need to have the next generation of architects and engineers thoroughly understand this and put it into practice."

Walking down this "solar street" is like a glimpse into the future.

Every house is its own electrical conductor. Walls, floors, windows and roofs are designed to absorb and produce the energy needed to run the house.

Some walls and roofs are covered with low-growing, drought-tolerant plants to attract birds and butterflies while insulating the house, reducing noise pollution, improving air quality and collecting rainwater. Water collected from the sink, shower and washing machine can be used in the garden or for washing a car — electric, of course.

Sustainable materials decorate the interiors, such as pressed wheat and sunflower hull cabinetry, recycled glass tiles and fly ash countertops made from the ash of incinerated garbage. Bamboo floors replace pine or oak because the durable, fast-growing grass yields four times as much material on the same amount of land.

The houses will be judged by industry professionals in 10 contests that include architecture, engineering, market viability and comfort. The houses will also be judged on energy-efficient lighting, heating/cooling, powering appliances and how much excess energy they have to power an electric car. The winner will receive a trophy, but the real prize is the recognition.

The schools range from two-time winner the University of Colorado to California's Santa Clara University, which King calls the "underdog." Santa Clara is the smallest team from the smallest school.

And its trip to Washington got off to a bumpy start when the truck transporting the house cross-country broke down in Nebraska. Although a hurried repair was arranged, the house didn't arrive until three days after construction started for everyone else.

The students worked non-stop to catch up, and Agustin Fonts, the electrical and construction coordinator, says they don't feel like the underdog. Mostly engineering majors, the Santa Clara students have confidence in their house, with its insulating glass wall and unique bamboo structural beams.

After D.C., many of the solar houses will become education centers, while others will take root as private homes. The University of Missouri-Rolla will turn its entry into student housing, as it has with the past two decathlon houses.

Past decathletes have gone to work for the DOE, state energy offices, architectural firms, construction firms and energy providers.

"You can't help but be enthused by walking through the solar village and talking to these kids," Energy Secretary Samuel Bodman says. "This event highlights the ingenuity and environmental aspects of solar energy, and part of the job here is to stimulate future engineers and architects."