The Maryland state insurance commissioner yesterday ordered Blue Cross and Blue Shield of Maryland to refund $15 million in overcharges to customers.

Commissioner Edward J. Birrane Jr. issued the order, which will affect 1.6 million policyholders, after the companies' annual statements for 1977 showed a surplus of $33.6 million in the Blue Cross reserve fund and $14.1 million in Blue Shield's reserve fund.

Birrane said yesterday that the companies will be required to submit plans for distributing the money by the end of the month, and until then it will be impossible to compute how large a refund an individual subscriber could expect. "It will depend on the amount of coverage someone has and on the distribution plan the companies submit," he said.

If the money were to be evenly distributed among policyholders, an average subscriber would receive a refund of about $9.

A Blue Cross-Blue Shield spokesman said the companies had no major objections to the order. "We did this on our own several years ago and were in the process of investigating the possibility of doing it again. The order is consistent with the way we are thinking."

Blue Cross, which covers hospital expenses for subscribers, and Blue Shield, which covers doctors bills, maintain reserve funds to protect them in the event of a disaster that would result in numerous claims at once.

The companies have 30 days to appeal Birrane's order in the Baltimore city courts, but a Blue Cross-Blue Shield spokesman said no appeal is planned.

The companies were ordered to begin mailing checks to subscribers no later than Sept. 1. Blue Cross must refund $12.5 million and Blue Shield $2.5 million.

Of the $15 million about $13 million will be distributed to over 7,000 group subscribers and about $2 million will go to individual subscribers, according to Birrane.

The commissioner he believes the first refund order to be in Maryland. He said he plans to examine Blue Cross-Blue Shield reserve funds every six months to see if a refund is in order.

Between 1972 and 1974 the two companies - which are nonprofit organizations - voluntarily refuned $20 million to subscribers, spokesman Nicholas Greaves said yesterday.

"The recent improvement in our financial condition should provide us with a welcome opportunity to redistribute some of our earnings among our members," Blue Cross-Blue Shield said in a written statement. "Our plans are under development and will be presented to the commissioner shortly."

Greaves said the companies had been told by consultants that they should maintain reserve funds to cover at least two months worth of claims and operating expenses.

"Currently we're a little bit under that," he said. "But at this rate we anticipate exceeding that figure by the end of 1978."

Federal employes covered by Blue Cross-Blue Shield will not be affected by the refunds because they have already received refunds. Federal groups have a refund clause written into their contracts with the companies, Birrane said.

"The important thing is that the subscribers receive their money in cash and not through any other method," Birrane said. "If they paid too much they deserve to receive the excess back in the form in which they paid it - a cash refund."

The reserve funds were apparently built up because fewer claims were submitted than had been anticipated. After discovering the surplus in the companies' annual reports. Birrane ordered a public hearing with company representatives June 2.

"I then met with staff to discuss the matter." he said. "The big question was how to distribute the money, over a long period of time or immediately."

Birrane said he hopes the refund checks can be mailed out over no more than two months.

Blue Cross represented more than 85 million customers nationally during 1977 and Blue Shield represented more than 72 million.