The D.C. Public Service Commission issued a final order yesterday granting an overall rate increase of 8 percent to the Washington Gas Light Co. that the company said will actually reduce the bills for up to 51,000 of its smallest customers.

The new rates went into effect immediately, a company spokesman said, and will be reflected in adjustments to the next bills sent to 150,000 residential and commercial customers in the District of Columbia. Customers in Maryland and Virginia are not affected.

Each of the company's 87,000 residential heating customers will pay $21.84 a year additional on his basic bills, the company estimated. The average monthly bill will go from $36.05 to $37.87. In reality, there are no such average bills, however, since they surge upward in the winter heating season.

The 51,000 customers who use gas only for cooking and water heating -- and who already pay the smallest bills -- will have their rates cut an average of 42 cents a month, from $10.09 to $9.67, the company reported.

This results from a reduction from $3.75 to $2.50 in the monthly service charge levied on such customers, whether they use any gas or not. The service charge for heating customers will drop from $8 to $5.

In a sharply worded dissent to the order signed by the other two PSC members, commissioner William R. Stratton said the small users will be carried on the backs of other customers.

Although overall revenue from D.C. operations will rise by 8 percent under the new rates, the impact will be felt unevenly be various groups of customers.

Collections from residential heating customers will rise 6.23 percent, while those from nonheating customers will fall 4.36 percent.

Other increases are: Commercial and industrial heating, 11.03 percent; commercial and industrial nonheating, 12.97 percent, and centrally metered apartments (in which the landlord or condominium organization pays a single bill), 13.95 percent.

The overall 8 percent increase is expected to produce $7.2 million in new revenue for the company, which had asked for a 12.7 percent increase producing $10.9 million.

A company spokesman said the firm was "appalled" by final approval of the lower figure, and said the rate case had dragged out sine 1977. In its company's needs with the customers' interests. A preliminary order proposing the rate adopted yesterday was issued by the PSC Feb. 13.

Increases went into effect in Virginia last October and in Maryland in January.