The D.C. Public Service Commission yesterday ordered a 6.8 percent increase in rates for Washington Gas Light Co. that will cost District customers $14.3 million a year, starting March 5.

WGL wanted more. The company asked last April for a 13.2 percent annual rate boost, or $26 million more each year.

Under the increase approved yesterday, the average monthly bill for a single-family home using gas heat will rise about 8 percent to $91 from an estimated average of $85, according to commission officials.

Single-family residential customers using gas for cooking and water heating--but not heat--will pay an average 8.7 percent more each month, or $35.90, compared with about $33 currently.

Apartment dwellers using gas for cooking and hot water would pay an average 11.4 percent more each month, or $9.89, compared with a current average monthly rate of $8.88.

WGL spokesman Paul Young said those who use less are being asked to pay more, proportionately, because the old rates did not cover the full cost of service.

"Basically, this is an attempt to try to bring various classes more in line with what it costs to serve them," Young said. "Some of those classes really weren't paying the full cost."

The approved rate increases will affect 130,000 residential customers and about 20,000 business customers in Washington, Young said. The average rate for business customers is difficult to estimate "because there simply is no such animal as an average business customer" in terms of gas usage, according to a commission spokesman.

WGL has argued that it needs higher rates to offset rising expenses, including labor costs.

Yesterday's rate increase comes at a time when gas bills already are growing rapidly because of natural gas deregulation. Increases in gas prices resulting from deregulation are passed on to consumers automatically through the "purchased gas adjustment" charge and are not limited by the PSC.

Last year the company received an $8.1 million, 5.1 percent, annual increase in Washington. A WGL increase of $9.4 million, 4 percent, is scheduled to take effect for the company's Maryland customers next month. WGL's last rate boost in Virginia came in 1981--a 2 percent raise generating $2.9 million annually.