Washington Gas Light Co. has redesigned its bills to make them easier to understand.

And now that some customers know what they're paying for, they're not happy about it.

The new statements spell out the "system charge," a fixed overhead fee that includes meter reading and billing. In the District of Columbia, the fee is called a "customer charge."

On the old bills, the charges were coded "SC" in Maryland and Virginia and "CC" in the District. The fees ranged from $2.50 to $9.55, depending on the jurisdiction and whether the customer used gas for heating.

In the past, some customers apparently thought that the codes meant fuel surcharges, even though the codes were explained on the back of the bill. Then, when the revamped statements spelling out "customer charge" arrived in the mail, it appeared to some to be a new fee that had just been added.

For those with small gas bills, the charge seemed worse.

One Washington woman, accustomed to paying $3 to $4 a month for gas cooking in her apartment, noticed the $2.50 "customer charge" for the first time when her new statement arrived this week. She was sure WGL had raised its rates.

So were some other residents in the woman's downtown apartment building. "Three elderly women came by with a petition protesting the rate increase on their bills . . . and I signed it," said the woman, who works as a switchboard operator.

The D.C. Public Service Commission reported that "quite a number" of people had called to complain about the "customer charge" on the new statements and to find out what it was.

In fact, said Washington Gas Light representative Susan Butz, the charges have been on the statements for months. Moreover, the charge for District residents is lower now than it was before the PSC ordered a reduction in March 1979.

The current system charges for WGL's 545,000 customers include:

In the District, $2.50 each month for nonheating residential customers. Heating customers pay $5 each month from September through May, but no fee the rest of the year. (The rate before March 1979 was $3.75 for non-heating customers and $8 for heating.)

In Maryland, $4.35 each month for nonheating customers and $6.15 each month for heating customers.

In Virginia, $3.55 each month for nonheating customers and $6.10 each month from September through May for low-volume heating customers. High-volume heating customers (those using 1,000 therms or more during winter months, with some annual adjustments) pay $9.55 each month from September through May. Heating customers don't pay a system charge during summer.

The charges vary from one jurisdiction to another in accordance with local regulations. "We propose a charge based on the cost of providing service divided by the number of customers in that group [such as residential]," said Butz. "But the public service commissioners have their own ideas and they may decide that they don't want a high system charge."

District charges, which are the lowest among the three jurisdictions, could increase if the utility wins its request for a $17.4 million rate increase. The case is pending now before the PSC.

Included in the system or customer charge are such expenses as taxes and depreciation and maintenance of company equipment on the customer's property. Other company overhead expenses -- such as the cost of operating WGL plants -- is part of the "current gas usage charge" on the customer's bill.

When a local regulatory commission decides to keep WGL customer or system charges below the level proposed by the company, the difference is figured into the "current gas usage charge" as much as local regulation allows, Butz said.

Brian Lederer, the People's Counsel who represents District residents before the PSC, said that the new WGL bills are in keeping with the Consumer's Bill of Rights approved by the District. "With this new bill, it is easier for customers to know what each charge is for," he said.

WGL spent months working on the design of the new statement, and company officials met twice with representatives of consumer groups for their suggestions. The new statements tell whether the bill is based on an actual company meter reading or if it was estimated. The bills also list the date the statement was mailed and when the next meter reading date will be. And the company now encloses an envelope to help customers pay by mail.

"Ninety-nine percent of the calls we hve received about the new statements have been favorable," Butz said.