US cable Internet customers are using an average of 268.7GB per month, and 4.1 percent of households use at least 1TB, according to new research by the vendor OpenVault.

Households that use at least 1TB a month are at risk of paying overage fees because of the 1TB data caps imposed by Comcast and other ISPs. Terabyte users nearly doubled year over year, as just 2.1 percent of households hit the 1TB mark last year, according to OpenVault.

Cable Internet providers use OpenVault products to track "broadband data usage consumption levels for millions of subscribers," the company says. This gives OpenVault visibility into how much data broadband customers use each month.

OpenVault found that households that face data caps use 8.5 percent less data than un-capped users, suggesting that cable customers limit their Internet usage when they face the prospect of overage fees. According to OpenVault, the caps can help cable companies avoid major network upgrades.

For cable Internet users, the need to limit usage to avoid overage fees isn't a selling point. But for OpenVault's cable industry customers, the ability to impose caps is a plus because it helps cable companies delay network upgrades.

"Our analysis makes it clear that usage-based billing is among the most effective tools the industry has in managing consumption and reducing the need for massive capital expenditures," OpenVault Executive VP Josh Barstow said in the OpenVault announcement.

Specifically, "OpenVault's 2018 data also shows that average usage for households with flat-rate pricing was 282.1GB/HH, more than 9 percent higher than the 258.2GB/HH average usage for households on usage-based billing (UBB) plans," OpenVault wrote. Stated another way, customers facing caps and overage fees use 8.5-percent less data than un-capped customers.

Un-capped customers are, naturally, more likely to exceed a terabyte. "The percentage of flat-rate (non-UBB) households exceeding 1TB of usage was 4.82 percent, a full percentage point higher than the 3.81 percent of UBB households who exceeded the 1TB threshold," OpenVault said.

Median usage rises 40%, year over year

OpenVault's new report is based on household usage in December 2018. The data comes entirely from cable networks, so it does not include any fiber, DSL, or wireless Internet services, an OpenVault spokesperson told Ars. OpenVault declined to say how many households were included in the data, and it's not clear which cable provider networks were studied.

The 268.7GB average household data used in December 2018 was "up from 226.4GB/HH [household] at the end of June 2018 and a 33.3 percent increase over the YE 2017 average of 201.6GB/HH," OpenVault said.

Median usage was 145.2GB in December 2018, "up from 116.4GB/HH in June 2018 and a 40 percent increase over the YE 2017 median of 103.6GB/HH," the company also said.

These numbers are in the general ballpark of what Comcast reports. Comcast says that "[a]s of June 2018, Xfinity Internet customers' median monthly data usage was 151GB per month during the past six months."

But while Comcast says that "more than 99 percent of our customers do not use 1 terabyte of data," OpenVault's research found a much higher percentage of customers exceeding 1TB. (Again, we don't know which cable networks were included in OpenVault's measurements.)

"The percentage of power users—defined as those households using 1TB or more—almost doubled in 2018, rising to 4.12 percent of all households from 2.11 percent in 2017, while the percentage of households exceeding 250GB rose to 36.4 percent from 28.4 percent during the same time span," OpenVault said.

Comcast and AT&T still impose data caps

Comcast imposed 300GB data caps in 2012, and raised the monthly cap to a terabyte in 2016. Customers who go over 1TB are charged $10 for each additional block of 50GB, up to a maximum of $200 a month. Comcast lets customers avoid overage fees by purchasing unlimited data for an extra $50 a month.

Charter, the second-largest home Internet provider in the US, is prohibited from selling plans with data caps and overage fees until 2023 thanks to merger conditions imposed on its 2016 purchase of Time Warner Cable.

But Comcast isn't the only major home Internet provider imposing caps and overage fees. AT&T, the third-largest home Internet provider, imposes caps ranging from 150GB to 1TB a month and charges overage fees of $10 for each additional 50GB. AT&T customers can get unlimited data by purchasing the gigabit speed tier, by bundling AT&T Internet with TV service, or by paying $30 extra per month.

Disclosure: The Advance/Newhouse Partnership, which owns 13 percent of Charter, is part of Advance Publications. Advance Publications owns Condé Nast, which owns Ars Technica.