TUCKED into a hillside 16 miles west of Aspen, Colo., is the headquarters of the Rocky Mountain Institute, also the residence of Hunter and Amory Lovins.The 4,000 square-foot experimental building is bright, spacious, and comfortable. It makes use of superinsulation, renewable energy sources, and a semitropical greenhouse to produce tomatoes and bananas. The heating and lighting bill is minuscule - despite a lengthy winter and mountainous setting where temperatures can drop to -40 F. The Lovinses live in one wing. The other wing is packed with desks, file cabinets, book cases, and computer equipment where RMI staff work. In the center is a large kitchen, dining space, and gathering area next to the "bioshelter" complete with fish pond (with aerating waterfall) and roaming iguana. With its 16-inch walls, argon-filled "Heat Mirror" windows (twice as efficient as triple glazing), solar design, compact florescent bulbs, and other energy-saving measures, the building averages just $5 a month in electric bills at the residential end and $15 a month in the work space (due to copiers and other business equipment). A pilot model refrigerator and freezer use no more than 15 percent the typical amount of electricity. When the outside temperature drops below freezing, refrigerator freon is p iped outside to take advantage of "passive cooling." Compressed-air showers and low-flow toilets conserve water. "We caulk the windows every year," says staff member Andrew Jones. "If we shut all the doors and windows and the sun stopped shining, we would only lose about nine-tenths of a degree [F] a day." The greenhouse acts as the building's "furnace," and the wood stoves, it turns out, are not really necessary. Even Nanuq, the resident bull terrier, counts as a source of energy. "She produces 65 to 70 watts of heat as she moves around the building," says Mr. Jones as he takes one of many visitors on a tour. "If you want to really get her cranked up to about 90 watts, throw in a ball." All of this state-of-the-art construction and equipment did not come cheap. The building cost $500,000, or about $130 a square-foot. But compared with usual construction for this area and conventional fuels, the building saves about $7,000 a year. This means the energy-saving devices paid for themselves in 10 months and will cover the full cost of the building in 40 years.