If Internet service providers' current experiments succeed, subscribers may end up paying for high-speed Internet based on how much material they download. Trials with such metered access, rather than the traditional monthly flat fee for unlimited connection time, offer enough bandwidth that they won't affect many consumers — yet.<p>But as more people use the Internet to watch TV and stream movies, they could bump up against the metered rates' caps, paying expensive over-use fees. Watching a movie may then require paying two fees: one for the movie, another to the cable company.<p>More and more television programming and movies are available online, through sites including Hulu, Netflix Watch Instantly, YouTube and Amazon.com's Video on Demand.<p>"This time there is a real, viable alternative" to cable, said Bobby Tulsiani, a senior analyst at Forrester Research.<p>Most consumers probably don't realize how much bandwidth their Internet usage consumes, because they've never had to care. Time Warner, the nation's third-largest Internet service provider, in its five experimental markets if offering five gigabytes of downloaded Internet content for $29.95 per month. That translates to 15 hours of viewing standard-definition video, or 350,000 e-mails, or 170 hours of online gaming, or some combination of those activities, according to the company. A high-definition movie consumes about 7 GB of bandwidth.