Washington Gas Light Co. reported yesterday that profits in the past year have declined substantially. But two other area firms engaged in the energy business -- Dynaelectron Corp. and Pargas Inc. -- said earnings rose in recent months.

The gas company, a Washington-based utility that distributes natural gas in the entire metropolitan area, said net income for the 12 months ended Sept. 30 fell to $9.78 million ($1.59 a share) from $14.78 million ($2.72) a year earlier. Revenues declined by $5.6 million to $349 million.

These figures don't include the effect of a change in accounting procedures to record revenues of gas delivered to customers but not yet billed. The cumulative effect of the change in the recent period was to add $1.01 a share to earnings, bringing final net income to $14.19 million ($12.60 a share). The company reports monthly on the most recent 12-month period.

A statement by Washington Gas attributed the decline in profitability to the "inadequacy" of a rate increase approved by the D.C. Public Service Commission last March and "delay" in regulatory action on the case, first filed in July 1977.

"Inflation and 9 percent warmer weather during the heating months of the current period also contributed to the earnings decline," WGL stated.

Dynaelectron's third-quarter earnings rose to $554,000 (8 cents a share) from a slim $6,000 in the 1978 period as sales grew to $88 million from $73.5 million. For the first nine months, the McLean company listed a rebound from losses of $987,000 in the 1978 period to profits of $1.5 million (22 cents). Revenues rose to $250 million from $214 million.

The company's hydrocarbon group reported higher earnings as Dynaelectron workers prepared to begin initial testing of a pilot plant in Kentucky that is scheduled to produce oil from coal. Dynaelectron also is engaged in commercial aviation services, technical consulting and electrical contracting.

Paragas, a Waldorf-based distributor of propane and owner of coal mines, reported record third-quarter profits of $2.1 million (58 cents a share) compared with a loss of $506,000 in the same period last year, as sales rose to $39 million from $28 million.

Nine-month profits of $7.7 million ($2.11) were more than four times the $1.6 million (41 cents) for the 1978 period and exceeded any prior full year's earnings in Pargas history. Profits last year were depressed by coal mining losses.

Chairman William Hill said that although wholesale propane prices have increased to record levels, Pargas has been able to reflect such cost rises in retail prices. Directors declared a quarterly dividend of 27 cents a share on common stock payable Nov. 29 to owners of record Nov. 15.