Washington Gas Light Co. will refund $7.5 million to its customers within the next 12 months - an average of $10 on each customer's annual residential heating bill.

The refunds, the result of lower rates charged by the gas company's major supplier, would reduce the average residential customer's bill from $550 a year to $540, according to a gas company spokesman. The refund will appear as a reduction in the purchased gas adjustment charge, the spokesman said.

The refund results from an order by the Federal Energy Regulatory Commission last month that said all of the rate increase requested by Columbia Gas Transmission Corp. was unwarranted, the spokesman said.

The rates had been in effect on a temporary basis since November 1974, when Columbia Gas requested them, the spokesman said.

"People say rates never go down," the Washington Gas spokesman said. "Here's a case where they are reduced."

The refunds will be apportioned among the gas company's three service areas according to the amount of gas sales in each area. Maryland customers will receive $3.39 million, Virginia customers, $2.15 million and District customers, $1.96 million.

In Baltimore, the Maryland Public Service Commission has ordered a $10 million refund to Baltimore Gas & Electric customers because the utility, following the commission's new formulas, had overbilled customers.

Those refunds will reduce the bill of a residential customer by $3.75 on the August billing only, a BG&G spokesman said.

Last October the commission designed a formula meant to control month-to-month bill fluctuations caused by utilities charging customers based on the prices they have to pay for fuel, the BG&E spokesman said.

However, the formula resulted in overcollections of $21,639,275 by June 1, the spokesman said. Only about half will be refunded, the spokesman added, because the commission felt that at some later time the utility may undercharge customers and therefore the situations would balance out.