Washington Gas Light Co.'s net earnings were off by $2.5 million for the latest 12-month period, and earnings for common stockholders were down $2.4 million, the utility company reported yesterday.

The gas company said its revenues for the year ended Oct. 31 increased to $428.4 million from $363.6 million in the same period a year ago.

But expenses increased almost as fast as revenues and interest charges were higher. As a result net income for the 12 months totaled $13.1 million down from $15.6 million. After paying preferred stock dividends, Washington gas reported common stockholders' earnings of $10.4 million ($2.36 a share) compared with $12.8 million ($2.92) a year earlier.

Washington Gas Light has changed its accounting methods in a way that tends to increase reported earnings for 1979; applying the change retroactively would decrease 1979 net to $9.8 million ($2.24) and show that 1980 earnings were up from the year before.

Baltimore Gas & Electric reported net earnings of $134.3 million ($3.50 a share) for the latest 12-month period, up from $127.1 million ($3.45) in the same period a year ago.

Revenues for Baltimore Gas totaled $1 billion for the 12 months, up from $841.6 million last year. A 2.4 percent increase in electricity sales revenues so far this year was due primarily to higher rates and unprecedented hot weather last summer, the company said. Natural gas sales, however, are running 1 percent behind 1979 levels.