eMarketer estimates that U.S. Internet TV users will reach 182.6 million this year, up 8.1 percent from 2017 and about 55 percent, according to a new forecast.

At the household level, eMarketer estimates that there are 88.7 million connected TELEVISION households in the United States, or 71.6 percent of all households.

Despite the high penetration rate, Internet TV advertising accounts for only a small part of overall TV advertising. "The trend for Internet TV advertising remains challenging due to differences between platforms and other factors," said Paul Verna, chief video analyst at eMarketer. "But because consumers are adopting Internet TV faster than many expect, it seems only a matter of time that advertising spending catches up."

According to estimates from Tru Optik of Broadcasting and Cable, advertising spending on U.S. Internet TV will total $8.2 billion this year, up about 75 percent from $4.7 billion in 2017. Tru Optik expects strong growth over the next two years, to $13.3 billion in 2019 and $20.1 billion in 2020.

Tru Optik predicts that TV advertising spending will account for about 12% in 2018. By contrast, eMarketer expects digital video advertising to reach $17.87 billion this year, accounting for 25.6 percent of television advertising spending and 16.7 percent for digital advertising. This includes video patch ads, as well as digital video ads in video players, including ads added in or after videos, but not on social media platforms such as Facebook or Twitter.

This shift in Internet TV is not limited to the U.S., with IHS Markit reporting that the share of smart TVs connected to the Internet will reach 70 percent of global TV shipments this year, up from 45 percent in 2015.