One in five U.S. households that use the Internet also watch television programming online, double the number of viewers from two years ago, a survey released Thursday showed.

The top two destinations for online broadcasts are the official TV channel homepages and YouTube.com, according to the study conducted by the Conference Board and market researcher TNS. The top reason for heading online for TV is to be able to watch broadcasts any time, rather than on a set schedule.

Other reasons include avoiding commercials and being able to watch from different locations. More than seven in 10 online TV watchers log on for entertainment on a daily basis, and one in 10 say entertainment is the most important Internet activity.

"The shift from appointment TV to content on demand is well underway," Michael Saxon, senior VP of brand and communications at TNS, said in a statement. "Fundamentally, consumers expect content to be available when they want it, and on the screen of their choice -- TV, PC, or mobile.

"For consumers, PCs enhance content on demand from simply time-shifting to place-shifting. Online content can be viewed in any room in the house, or at work or school."

The top five types of shows viewed online are news, which attracted 43% of Web TV watchers; drama, 39%; sitcom/comedy, 34%; reality shows, 23%; and sports, 16%. Fully 15% watch content generated by users of video-sharing sites, such as YouTube.com.

Nearly nine in 10 online TV viewers watch from home, while 15% watch from the office and 6% from other locations, including the library or a friend's home. Some viewers watch from multiple locations, which is why the numbers add up to more than 100%.

A total of 68% of online TV fans watch streaming video and 38% view free downloads. Official TV channel homepages attracted 65% of viewers and YouTube.com 41%. Other sites cited by survey respondents included Apple iTunes, Hulu, file-sharing and social-networking sites, and Limewire. Few consumers surveyed were willing to subscribe to pay-per-download services.

The quarterly study is based on a survey of 10,000 U.S. households. The latest poll was taken during the second quarter.