Taxpayers who have installed solar energy equipment in their homes since April 19, 1977, can file for a substantial tax credit this year.

But the question of what types of solar equipment will qualify and what will not has become the focus of a backroom controversy, and even at this late date the answer remains unclear.

The problem stems in part from ambiguities and contradictions in the legislation passed by the last Congress as part of the National Energy Act. And, according to solar advocates and Department of Energy sources, a bad situation is being made even worse by policy makers at the Department of Treasury who are interpreting the law in an "overly restrictive fashion."

As a result, most of the solar space-heating systems constructed so far in the United States may be ruled ineligible.

In its initial proposal for a solar tax credit, the White House dealt only with "active" solar systems.

These are the familiar box-like solar collectors that heat either air or water, which then is pumped into the house.

Passive design, the other basic approach to solar space heating, was not even considered. Passive systems are a collection of techniques that include south-facing windows, attached greenhouses, skylights and the use of thick heat-absorbing walls. Recent Energy Department studies have found properly designed passive systems to be extremely effective. And the latest surveys suggest that there may be more than twice as many passive solar heating systems as active ones in the nation today.

"Because they considered solar tax credits to besacrosanct, senior administrators here did not give them much attention, preferring to devote their time and effort to more controversial issues," an Energy Department staffer said.

In fact, the White House delegated lobbying jurisdiction on this issue to the Treasury Department, which had previously opposed the idea.

Members of the solar coalition -- congressmen who favor solar energy development -- tried to compensate for the administration's oversight by including in the law language that specified that the tax credits would apply to passive systems. But members of the Joint Economic Committee, with Treasury Department backing, raised the knotty question of how one could identify a passive solar system.

This is an issue that still nags solar energy experts. As a 1978 Department of Energy report on the subject states, "passive building design is too varied to be easily classified and generalized."

Moshe Schuldinger of the Treasury's tax policy office put it this way: "If a person put so many windows on the south side of his house, how would we know that he is doing this to save energy -- that he wouldn't do it even without the tax credit? If he was going to do it anyway for other reasons, tax credit becomes just a windfall."

Responding to this type of concern, the legislators specifically excluded any method of hear storage that serves another purpose, such as a swimming pool, from the National Energy Act.

In its accompanying commentary (which has the force of law) they instructed that solar components would have to serve a "signficant structural function which would likewise be eligible for tax credit."

The end result of this process was a contradictory piece of legislation. On the one hand, it specified that passive solar systems qualified for a tax credit. On the other hand, because passive systems generally are in integral part of a building, it virtually eliminated them.

"I think this unfortunate outcome was the result of ignorance on the part of the members of Congress who were involved. Perhaps two of them understood the issues," said Susannah Lawrence of the Solar Lobby, the group best known for its sponsorship of Sun Day.

After legislation like this passes, the Treasury Department must translate it into specific regulations for the taxpayers. The solar tax credit regulations have not yet been published. "We hope they will be out by April 15," said Walter Woo of the Internal Revenue Service, with a laugh. As a result, Treasury officials will not discuss specifics.

However, an early indication of the Treasury's philosophy was contained in an advisory to taxpayers publicshed last Novmber. This publication stated, "For the purposes of the credit, materials and components that serve a significant structural function or are structural components, such as extra-thick walls, windows, skylights, greenhouses and roof overhangs, are not included as solar energy property."

Essentially, policy makers at the Treasury Department are arguing that by "significant structural function" Congress meant any structural function. "Out interpretation is that, to qualify, solar equipment must be unique to that function," Schuldinger affirmed.

The publication of the interpretation produced shock waves that have spread through the solar energy community, lower echelons of the energy Department's solar energy division and the solar energy constituency in Congress.

According to a spokesman for Sen. Gary Hart (D-Colo.), the solar coalition will meet with President Carter concerning his soon-to-be-announced solar program. A tax credit issue will be high on their list of prorities.

For the last few months, Energy Department experts have been working with Treasury offocials in the attempt to get them to liberalize their interpretation of the law. However, this effort appears to have had little effect.

Solar coalition member hope to persuade the president to use his discreditonary powers to get the Treasury Department to loosen its restriction on passive systems. Traditionally, however, tax-regulation writers have resisted outside pressure, even from presidents.

"We must follow the statutes, whether they make sense or not," Schuldinger said.

"We think there will be a substantial push in Congress to amend this legislation," said Terry Johnson of Hart's office. "And we can always make the revisions retroactive."