Customers of Washington Gas Light Co. will benefit from a small refund starting this month, the end result of federal government complaints that a natural gas transmission firm overcharged for gas.

Washington Gas also reported an increase in profits for the 12 months ended May 31 but the earnings are still below levels permitted by area regulatory agencies, the company said.

The alleged overcharged by Columbia Gas Transmission Corp. occurred during the period from October 1976 through last April, according to the Federal Energy Regulatory Commission. The FERC ordered refunds and Washington Gas, a local distributor which buys from Columbia, will receive $5.1 million to pass along to its users.

Paul Young, a gas company spokesman, said yesterday the refund will appear on monthly bills as a credit of one-half cent a therm per month over 12 months - or between $7 and $8 over the year for average heating customers.

Washington Gas profits in the recent 12 months, reported separately, rose to $17.8 million ($3.40 a share) from $15.1 million ($2.79) in the previous comparable period. The year-earlier earnings included a litigation settlement equal to 32 cents a share.

Revenues of WGL were $359 million in the recent 12 months compared with $297 million. Company directors voted a regular quarterly divident of 57 cents a share on common stock, payable Aug. 1 to stockholders of record July 10. Meanwhile, other area firms reported earnings as follows:

Solon Automated Services, a large Southeast Washington service company that owns and operates coin laundry equipment, reported profits of $2.7 million (87 cents a share, diluted) in the six months ended March 31 compared with $1.4 million (47 cents) a year earlier. Revenues rose to $28.7 million from $22.9 million.

Second-quarter earnings jumped to $1.5 million (48 cents) from $400,000 (14 cents) and revenues increased to $15.2 million from $11.4 million. On Dec. 1, Solon acquired 82 percent of the Sugarbush ski resort in Warren, Vt. and the ownership share was boost to 100 percent in April. Sugarbush contributed $657,000 in earnings to Solon for the six months but normally operates at a loss in the summer season.