Time Warner Cable and Charter do not automatically offer refunds or credits to their overcharged customers, according to the results of a US Senate investigation published this week.

Comcast, DirecTV, and Dish, meanwhile, do offer refunds or credits for overcharges caused by company billing errors, the study found.

Time Warner and Charter billing errors were mostly overcharges for equipment that its customers rent, including set-top boxes and routers. A Senate subcommittee monitored customer accounts for more than six years, and found that the two companies "made no effort to trace equipment overcharges to their origin unless customers specifically asked them to and did not provide notice or refunds to customers."

The subcommittee estimates that TWC has already overbilled customers $639,948 between January and April, and will overbill customers $1,919,844 by the end of 2016. As a result of the investigation, Charter informed the subcommittee that it overbilled customers by at least $442,691 per month.

TWC said it corrects overcharges using an often buggy computer program. The company told the investigators that its aim is to correct only 80 percent of billing errors through the program. Last month, it did not correct any because the program crashed.

Although overbilling still frequently occurs at Comcast, DirectTV, and Dish, they employ more advanced practices to detect it. In Dish's case, the company has automatic protections that make it impossible for a customer's rental equipment to be added or removed without triggering a change in the billing record.

As a result of the investigation, TWC offered to provide an automatic one-month credit to all customers for each piece of overbilled equipment or service, while Charter has decided to give its customers a one-year credit for any equipment overcharges.

"Although both Time Warner Cable and Charter point to technical limitations that make it difficult to determine how long a customer has been overcharged, their competitors' systems demonstrate that it is possible to implement more effective policies," the report concludes.

At the subcommitee's hearing to discuss the report, a TWC representative said the company's billing woes favor customers, according to the Washington Post.

"We're undercharging customers significantly more than we're overcharging them," said John Keib, a hearing witness who was until recently Time Warner Cable's executive vice president of residential services.

The report comes after the Department of Justice and Federal Communications Commission in April greenlit Charter's $78 billion acquisition of TWC and the related $10.4 billion purchase of Bright House Networks.