From the top floor of the U.S. Environmental Protection Agency in southwest Washington you can see across the Potomac a 508-megawatt electric power plant burning 3,000 tons of coal a day in the heart of the metropolitan area.

Yet, not a wiff of smoke comes from its five stacks.

It is a tribute to American technology and to tough policies developed by the EAP and state agencies under the national Clean Air Act. It also is an enormously expensive feat, one that might have been avoided altogether had not clean air goals clashed with energy needs.

The Potomac Electric Power Co.'s Alexandria plant is one of six serving the utility's 512,000 customers. To curb the black smoke billowing from it in the mid-1970s, the company spent $43 million or $84 per customer to install five antipollution devices called hot gas precipitators.

These bulky contraptions attached to the back of the plant capture 300 tons of ash a day from the burning coal that otherwise would go out to the smokestacks. Trucks haul the ash to landfills.

The plant was built in 1949 for $69 million with modest pollution control equipment. When 1970's tough new clean air rules took effect, Pepco planned to convert the plant to oil, which burns cleaner than coal.

The conversion, planned for late 1973, never took place because the Arab oil embargo at that time made shockingly clear America's need to lessen its dependence on foreign oil.

Although it could not switch to oil, Pepco still had to meet clean air rules, so it changed the coal burned in the plant from 3 percent sulfur to 1 percent. This caused a new problem because the 1 percent coal contained more ash than the old antipollution equipment could handle.

"The equipment became overwhelmed," said John e. Rasmussen, Pepco's director of engineering and construction.

The plant began belching balck smoke.

"I used to see it and get sick to my stomach," said plant manager Robert M. Lewis. Alexandria residents complained bitterly. In 1975 Pepco began installing the new precipitators, work that was recently completed.

Now Rasmussen says the equipment removes 99.7 percent of the ash. Emissions from the smokestacks are still 10 percent opaque on a scale of zero to 100, but this is not visible. Virginia air quality officials say the plant's emissions meet state and federal standards.

The coal pile next to the plant -- a 100-day supply -- is watered down to keep the wind from blowing it. When the 42 coal cars roll in each day, they dump their contents in a building with dust-collection equipment.

Sometimes Lewis finds himself yearning for oil -- neater, cleaner, simple to handle. With coal, he said, "Sometimes it seems like you're fighting it all the time."