Within two years U.S. families may be able to install solar water heaters in their homes for less than one half the present cost.

The impetus may come from a program that is to be expanded from Phoenix to the rest of the country, Department of Energy officials announced last week.

An Arizona State University professor here was awarded a $600,000 grant by DOE to begin a two-year, $1 million national expansion of a local program that instructs homeowners how to construct and install their own solar heaters.

The program is seen as a boost to the solar industry and as a means of lessening donstic reliance on diminishing overseas fuel supplies.

"We think it's a super program because everybody you can think of benefits," said Stanley Lumma, a professor who has operated a pilot project at the university since 1978 using a $1,200 federal grant. "We spent a lot of time working on this."

The $600,000 grant will be used during the next year to train program instructors in 14 cities. These instructors will teach a 12-hour workshop that shows homeowners through lecture and demonstration how to construct and install their own solar water heaters. An additional $400,000, which officials said was tentatively approved, will expand the program to cities in all 50 states by the end of 1982.

The workshop costs about $50 and the necessary hardware is priced at about $800. State and federal tax credits cover up to 75 percent of the costs in some states. The average cost of a commercial solar water heating unit is $2,500 but can run as high as $4,000, Lumma and energy department officials said.

"What is particularly notable about this program is that it is self-supporting," explained energy department spokesman Rob Stern. "While allowing homeowners to build their own system, it will also encourage the commercial solar water heating industry.

"The more solar water heaters out there, the more that particular innovation will be adopted. These things spread by word-of-mouth for the most part.