A little noticed defense bill now in Congress might well prove the biggest boost for solar energy forces since President Carter's solar tax credit proposal.

The bill - authorizing $4 billion in military construction - would require solar heating and cooling systems for all new military family housing units and for 25 percent of other military construction.

At least $100 million a year would be spent by the Pentagon on solar systems, in an industry that had total sales of $150 million last year.

"These federal purchases will give the solar industry a long-needed shot in the arm," said Sen. Gary W. Hart (D-Colo.), sponsor of the solar provision and chairman of the Senate military construction and stockpites subcommittee.

"Besides conserving energy, large scale purchases by the Department of Defense will stimulate mass production of these systems and bring down the cost of solar for consumers," he said.

Solar energy systems will increase the initial cost of construction between 10 and 20 percent, Hart said. But the bill specifies that the military solar systems be "cost-effective" over their lives, returning more in fuel savings than the additional expense.

House-Senate conferees reached agreement on the authorization bill Tuesday night. The original House bill did not include the solar requirement, but the conferees accepted the provision without opposition.

If the solar energy requirement passes both houses - as expected - the solar industry would have to expand rapidly to fulfill demand.

Kathy Bushkin, press aide to Hart, acknowledged that the solar industry could not currently meet this increased demand, but said, "The manufacturers say they are ready and waiting. This is the kind of bill they need to really gear up."

Bushkin said Hart hoped that the military could do for solar energy what it once did for safety razors.

"The military literally transformed America's shaving habits, turning people from straight razors to safety razors, simply by buying enough of them. The military purchases can drive the unit cost down and help make America realize that this is something that is feasible and whose time has come.

Joe Overton, legislative assistant to Rep. Barry M. Goldwater Jr. (R-Calif), who has colosely watched solar energy bills, pointed out that "there is going to have to be tremendous capital investment if this bill is ratified."

"Most of the firms making solar equipment today are small, and most are not even semi-automated. You don't have much mass production," he said.

Although Overton agreed that the military requirement was "a healthy step for solar energy," he expressed concern about the maintenance and reliability of solar energy systems.

"There can be major maintenance problems that the military will have to be aware of," Overton said . "You also have a warranty problem."

Although most solar energy system are computed to need 10 to 15 years to pay back their additional cost through fuel savings, the average warranty for a solar energy unit is only three years.

"You are asking the consumer - in this case, the taxpayer - to carry a dang-high risk," Overton said.