The nation's solar industry grew by 25 percent during the past year, according to federal Energy Department, and the overwhelming majority of solar equipment is being used to heat hot water or swimming pools rather than homes.

Industry officials term the growth good but not great.

During 1979, the industry delivered 14.3 million square feet of solar collectors -- an increase of 3.5 million square feet over 1978 -- according to an Energy Department survey of solar manufacturers.

"I think there's been reasonable growth," says Bob Jordan, a top solar official in the department. "There are a lot of impediments to solar; some have been eliminated, and others we're still trying to eliminate."

Congress has enacted a solar tax credit and currently is considering increasing it and providing a "solar bank" to provide low-interest loans for solar installations.

The Solar Energy Industries Association estimates that 250,000 homes had either solar hot water heating, pool heating or space heating at the end of 1979, and predicts there will be some some 341,000 such solar homes by the end of 1980.

"It's not the type of industry where we're going to have an explosive growth," Randy Dyer, an association official. "People have to retrofit their homes, and there's a long process of public education."

Officials seem relatively content with the industry's growth, but they acknowledge that the solar industry remains a relative small one.

"while there are some 50,000 solar hot water systems produced each year, the number of gas and electrict hot water heaters produced is some 5 million," said David Hartman, a solar market researcher for Booz-Allen & Hamilton, a private consulting firm.

Hartman says that without a greater effort, the nation will have serious trouble achieving the National Energy Plan goal of 2.5 million solar homes by 1985.

He notes that the industry has the capacity to greatly increase its production, and says a stronger market is needed if firms -- particularly the bigger ones -- are to make a stronger commitment to manufacturing and marketing solar products.

So far, the major application of solar energy has been to heat swimming pools and domestic hot water. More than 80 percent of the solar collectors shipped during the past year were for these purposes.