While solar energy is free, quiet, clean and "magical," it is "not now commercial" for most home installations, a veteran engineer in the housing field said this week.

"It's mainly a matter of fully tested equipment and the capital investments required, plus standards and warranties for performance," engineer Ralph Johnson told members of the National Association of Home Builders meeting here. Johnson is president of the NAHB Research Foundation in Rockville.

Johnson - who attended a meeting of the International Solar Society 23 years ago, when the cost of energy was not a major issue - said that the escalated costs of gas, oil and coal have brought solar adaptations into the spotlight "with some of the most exciting ideas since the horseless carriage."

Johnson estimated that only 10,000 U.S. homes now are heated primarily by solar systems but he said the number is growing.

"With air or water heat transfer installations costing about $12,500 today, it's not likely that most Americans can afford solar heat as an investment on an economic basis," he said. "While it's not around the corner, it's on the way. Now it seems to be much more reasonable try back-up solar heating for the production of hot water used in a house."