Boston — Solar energy consultant Charles Vidich describes a recent encounter with a homeowner who had installed a sun-powered water heating system: ''I asked, aren't you worried that your system could be shaded?''
''Why should I be worried?'' replied the homeowner. ''There's got to be some law that says there's a right to light.''
That, says Mr. Vidich, is just the point. In most communities across the US such a law has yet to be written. ''People are making their decisions on that kind of gut level thinking - that there's a perfect right to it (sunlight) once they put it in,'' he observes. ''But that's just not the way it is.''
To Mr. Vidich, who works for a regional planning agency in Connecticut, and to many others in the solar field, the message is clear. If solar energy is ever to fulfill the bright promise many people see for it, it is going to need some legal protection.
That is the view of experts whose work has brought them head-to-head with barriers to building passive solar houses or fitting existing structures with solar facilities.
Their findings: With few exceptions, states and municipalities have yet to see a need for laws or zoning ordinances guaranteeing access to the sun. Moreover, some of the state laws designed to protect solar access are criticized as being weak.
As a result, almost anyone who can win local approval to build or expand a structure higher than the one on an adjoining property to the north can block the sun from a solar unit and render it almost useless. Even a tree that grows high enough over the years can shade such a system.
The problem takes on new significance as city planners focus on increasing densities to make their communities more energy-efficient.
Looking beyond the current slump in its industry and to the trend toward energy efficiency, the National Association of Home Builders forecasts that by 1985 one-fourth of all new housing starts in the United States will be of passive solar design.
A November 1980 Gallup poll found that a majority of respondents thought passive solar design or solar-heated domestic water should be required by law in new home construction. Nearly half of those who favored this approach thought local ordinances would be more effective in promoting it than would state or federal laws.
But the battle is barely joined, much less won.
Several states have passed or are considering solar access protection laws, including California, New Mexico, New Jersey, and Connecticut. But, says one source who follows solar issues closely: ''Really, all (most of them) say is that you can buy an easement from your neighbor. You don't need to pass a law for that; it already exists in common law. You're just putting the burden on the person who wants to put up a solar collector. Solar access is a land-use problem. A law shouldn't just say you've got a right to light.''
The New Mexico solar access law, in particular, is frequently likened to the old water-rights statutes of the West: that is, first come, first served.
Attempting to benefit from the experiences of other states, Wisconsin state Rep. Sharon Metz (D) of Green Bay has led the fight for passage of a bill that would require local governments to consider applications for solar access permits. The bill has passed the lower house of the Wisconsin Legislature, but in such watered-down form that her legislative aide, John Peacock, calls it only ''a small step in the right direction.'' He hopes the Senate can be persuaded to strengthen it.
''We're trying not to take the easy way out,'' Mr. Peacock says.
Among smaller units of government, Albuquerque, N.M., is one of the leaders in safeguarding solar access. The city has an ordinance that prohibits any structure from casting a shadow two feet above the mean grade of the property to the north in certain zoning districts.
One town faced with the twin issues of increasing density and solar-access protection is Oxford, Conn., near Waterbury. The town emerged from the 1980 census as No. 2 in the state in percentage of growth.
Oxford town planner Joseph Devonshuk says a proposed solar-access ordinance already has cleared the planning and zoning commissions and is scheduled for a public hearing in mid-January. Little opposition is expected, he says, adding that most of the resistance comes from persons who don't understand the implications of the issue.
Advocates of solar energy say the need for legal protection is doubly important because the few court tests on record have resulted in decisions unfavorable to their cause.
Glenn Prah, a Muskego, Wis., homeowner can testify to the difficulty of taking solar-access cases to litigation. In September 1980, a county court refused to hear his suit against a neighbor whose new house shades part of Mr. Prah's sophisticated $18,000 active solar unit. The reason: No local zoning ordinances protecting solar access were on the books. Prah tried again before a district court. It bucked the matter to the state Supreme Court, which agreed in October to rule on whether the case can be heard. If the ruling is in his favor, Prah will renew his effort to collect damages.
Muskego, a Milwaukee suburb, now has a solar-access protection ordinance, but too late to help Prah. In the meantime, he says, he has spent another $2,000 to modify his unit so it will function properly in Wisconsin's frigid winter weather.