If videoconferencing is compared to a plane trip, what's the distance travelled? Is the plane full or not? In what year was it built? On the other hand, how long does the videoconference take? Does it happen over a wired or a wireless access network? Do you use a laptop or a high-end telepresence system? When you're streaming music, do you listen to a song once or twenty times? If you buy a DVD, do you go to the store by car or by bike? How long is the trip? Do you only buy the DVD or do you also shop for other stuff?

One important difference between online and offline services is the role of time. Online, energy use increases with the time of the activity. If you read two articles instead of one article on a digital news site, you consume more energy. But if you buy a newspaper, the energy use is independent of the number of articles you read. A newspaper could even be read by two people so that energy use per person is halved.

A sensitivity analysis generates very different conclusions from the ones that are usually presented. For example: streaming a music album over the internet 27 times can use more energy than the manufacturing and transportation of its CD equivalent. 25 Or, reading a digital newspaper on a desktop PC uses more energy than reading a paper version from the moment the reading length exceeds one hour and a quarter, taking the view that the newspaper is read by one person. 26 Or, in the earlier mentioned study about the energy advantage of videoconferencing, reducing the international participant's travel distance from 5,000 to 333 km makes travelling in person more energy efficient than videoconferencing when a high-end telepresence system is used. Similarly, if the online conference takes not 5 but 75 hours, it's more energy efficient to fly 5,000 km.