American consumers have made conservation a habit and grown accustomed to colder homes. So says a study released here by Cambridge Energy Research Associates. The study said the total amount of energy used in US homes was lower now than in 1973 - despite increases in both population and the number of appliances in use. Improved furnace maintenance, weatherstripping, and use of supplemental heating systems helped curb energy use, but the study said Americans definitely lived in colder homes. A decade ago 85 percent of the population kept their dwellings at 70 degrees or warmer during the winter. Now, only 45 percent keep their homes that warm, the researchers said.