Walter F. Szymczewski proudly showed off the windmill he recently built for $7,000 in the back yard of his Maryland home. It was a hot, muggy afternoon and the windmill was not turning because there was no wind. There had not been any for some days.

This did not bother Szymczewski. He built the four-blade, 30-foot-high machine for fun as an experiment -- "took the gamble to find out what it would actually do." In fact, it has shaved a little off his electricity bill already, even though summer is the worst possible season in the Washington area for wind and windmills.

"This cloudy, rainy weather is bad. All you get is these up and down drafts, these little spotty currents," Szymczewski said. "That won't drive a windmill. You need a whole weather front movement. Usually that begins in late September, and the wind is steadier all through the winter."

Over the course of a year, Szymczewski hopes, the wind will provide one-third to one-half the electric power used in his modest house in Severn, Md.

To the west of Washington in Clarksburg, Md., Montgomery County police officer Karl W. Plitt, 34, installed a sleek, white Enertech windmill on a 50-foot telephone pole in his back yard. Results so far: a $90 saving on his electricity bill in two months.

Small-time windmill experimenters like these are sprinkled throughout the Washington area, and some energy experts believe they are in the vanguard of what will become a significant national effort to reap the wind.

A new study by Worldwatch Institute, a Washington think tank, estimates that wind power could supply 20-to-30 percent of the electricity in many countries by the early part of the next century. The study said that there are 3.8 million homes and 370,000 farms in areas of the United States where the wind is strong and steady enough to make wind-powered electric generation feasible.

In some of those areas, utility companies are taking an early interest in wind power. Southern California Edison, for example, has set a corporate goal of generating 30 percent of its electricity from such sources as sun and wind, according to Douglas C. Bauer, senior vice president of the Edison Electric Institute, which represents utility companies.

Bauer said other companies are examining the potential of wind power, but are concerned about its reliability as a new technology fueled by sometimes fickle winds.

While man has used windmills for grinding corn and drawing water for hundreds of years, electricity-generating windmills that utility companies plan to use in groups on "windfarms" are so huge that they present new technological problems.

Operations at a huge U.S. Department of Energy experimental windmill in North Carolina, for example, had to be curtailed after nearby residents complained that the 200-foot blades on the 2,000-kilowatt machine made an annoying swishing noise.

The winds in Washington generally are not steady and strong enough to justify installing a windmill, according to Energy Department spokesman Jay Vivari.

"The most important thing is to find an area with a high annual mean wind speed," he said. "Chicago, the windy city, is lousy. The wind blows like mad but it's not good on an annual basis. All through the Appalachians is good . . . all the way up through New England . . . A dynamite area is the Texas panhandle, western Kansas, central Wyoming."

Vivari said that as a rule of thumb you need an annual average wind speed of 10 knots (roughly 11.5 m.p.h.) for a windmill to be a good investment. The highest such wind speed in Maryland listed in "The Wind Power Book," by Jack Park, is 9.6 knots at Baltimore-Washington International Airport. The figure at Fort Meade, which is near Szymczewski's house, is only 4.4 knots. The highest figure listed in Virginia is 8.8 knots at Norfolk, and in the District of Columbia, 7.5 knots at Bolling Air Force Base.

To encourage the development of alternative energy sources, Congress authorized a 40 percent tax credit for the purchase and installation of windmills -- in Szymczewski's case, worth $2,800, which he is able to take over two tax years.

It is not clear, however, how much longer that will last. Reagan administration economists are thinking of trying to eliminate the credit in order to increase tax revenues. They are also seeking to slash DOE outlays on wind energy promotion and development, although the Energy Department will continue to do research in the area, according to Vivari.

The Reagan theory is that if wind power is economically feasible, the private sector will develop it with little aid from government.

In another earlier effort to encourage the development of wind power, Congress in 1978 mandated that utility companies must allow individual home owners and other small wind-power originators to hook into their power grids and must pay them a fair rate for any extra electricity generated. Public service commissions in the Washington area and most other parts of the country are still deciding the terms under which these transactions will take place.

So far, four big electric utilities in the Washington-Baltimore area report only six of their millions of customers have installed windmills and wired them into the utility company power grids since the 1978 congressional action.

Potomac Edison Co.'s Donald Whipp said: "It's in the experimental stage. It's too early to make any assumptions on the ultimate impact."

Szymczewski is not one of the six. His house is wired to receive electricity from Baltimore Gas & Electric Co., or he can shut off that power source and use his own windmill-generated power, which he stores in more than 200 batteries. Because his windmill is not wired into the BG&E power grid, Szymczewski cannot use the two sources simultaneously, nor can he sell his excess power to the company.

It was this simultaneous use and buyback that the 1978 congressional legislation made possible. The idea was to eliminate the need for expensive storage batteries, which were needed to make home owner windmill systems run smoothly but the cost of which was prohibitive.

Szymczewski, 66, a retired precision sheet-metal worker, was lucky and bought his batteries cheap when the telephone company got rid of them. New, they would have cost him more than $40,000.

Plitt, the Montgomery County police officer, does have his windmill wired into the Potomac Edison power grid. This means that when his windmill is working and delivering power to the house, Plitt's Potomac Edison electric meter runs slower. If a storm drove the windmill furiously at night when Plitt's house was using little power, the electric meter might actually run backward, although this has not happened yet.

Plitt bought his house in a subdivision in rural Clarksburg with wind power in mind. The modern, comfortable house is situated near the top of a high ridge at the end of a long valley -- "an ideal wind site."

The Plitts paid $10,000 for their sleek machine, which is perched atop a 50-foot telephone pole in their back yard, and for a solar hot-water system that complements it. After the tax credit, the total bill came to only $6,000 for the two systems, and a study of their first electricity bill indicates that they saved about $90 already -- or half what the two-month bill would have been.

The house has electric heating, which in the past cost $300 every two months during the winter. Now they hope these bills will be under $100 since winter is a time when the wind across their ridge blows strong and steady.

The Plitts' machine was manufactured by Enertech, a Vermont firm that has sold more than 700 windmills to become the country's largest manufacturer of electricity-generating windmills.

Enertech board chairman Ned Coffin said a 1.8 kilowatt machine like the Plitts' should provide 200-to-500 kilowatt hours of electricity a month at a good wind site. The average American home uses about 1,000 kilowatt hours a month.

Coffin said Bendix Corp. recently bought a 30 percent share of the fledgling windmill company.

Plitt discovered one disturbing aspect of his new relationship with the power company: because of safety requirements, he cannot use his windmill when the power company system fails in a storm or for some other reason. This is to prevent the windmill from "backfeeding" power into the company system and electrocuting line repairmen.

It is a problem for Plitt because having a backup system for power outages was one thought behind his getting a windmill in the first place. With some expensive rewiring that will prevent backfeeding, Plitt thinks he can solve the problem to the company's satisfaction.

All the big Washington-area electric utilitiies -- Potomac Electric Power Co., Virginia Electric and Power Co., Potomac Edison and BG&E -- have this same safety requirement.

The desire for energy independence goes beyond windmills for both Plitt and Szymczewski. They both have wood stoves and Szymczewski has not paid a cent for heating oil in five years. He gathers his firewood for free.

"This country is in trouble, that's why I had to do this," he said. "I was priced out of the heating market. There's nothing new about wind generators . . . You just have to have an interest in wanting to better your environment, living conditions. A hundred years ago, every man had to feed his family off the land."