Suburban Maryland's water commission yesterday voted to increased annual water and sewer bills by an average of $16.38 per household because people in Mongomery have not been using enough water.

Faced with sagging revenue due to the long, soggy summer, the Washington Suburban Sanitary Commission unanimously increase for residental customers.

The commission denied its widely publicized conservation campaign was responsible for lagging water sales. It blamed heavenly downpours that left lawn sprinklers idle.

"We can't control an act of God," said WSSC Chairman David R. Scotten in denfense of the rate rise scheduled to take effect Dec. 1.

The agenecy, which provides water and sewer services for most of Prince George's and Montogomery counties, said its 1.2 million customers -- cursed with the wettest weekends in recent memory -- had not been watering their lawns enough.

So to make up for the projected $5.4 million WSSC deficit, the conservation-conscious consumer will be asked to pay for the plentiful rains.

"It does appear that way," Montgomery County government spokesman Charles A. Maier said yesterday. "It doesn't make sense."

During a brief meeting with the commissioners, WSSC general manager Robert S. McGarry proposed the increase, which is expected to add $5.5 million to the agency's coffers. Combined with a $2.6 million projected decrease in expenses, the agency hopes to obtain a surplus by June 1980.

The rate increase works out to about $4.58 per person or just over $4 per quarter for the average household, which uses 300 gallons of water a day. The average yearly water bill would rise from $226.72 to $243.10.

The commission decided to schedule a public hearing on the subject Nov. 7 before formally putting the increase into effect.One commissioner at yesterday's hearing wondered aloud it public comments "would really make a difference."

According to WSSC statistics, water-sewer customers are using only 135 million gallons of water a day rather than the 140 million gallons the commission forecast.

The summer's heavy rainfall threw those figures off, the WSSC said, McGarry pointed out yesterday that waste water increased as a result of Hurricane David, which added to the cost of maintaining sewer services while water sales were declining.

McGarry noted that consumers who conserve water, using less than 300 gallons a day, will not have as high a water bill.

That confused many spectators, one of whom asked the commission if the rates would be raised yet again because water sales would decrease. The question went unanswered.

Commission member Johanna S. Norris, who said yesterday it was painful for her to vote the rate rise, told McGarry to consider a hiring freeze to keep the costs down.

"We have to show the general public," Norris said, "that we are tightening our belts."