  In recent years, the global use of online video services has increased
rapidly. Today, a manifold of applications, such as video streaming, video
conferencing, live broadcasting, and social networks, make use of this
technology. A recent study found that the development and the success of these
services had as a consequence that, nowadays, more than 1% of the global
greenhouse-gas emissions are related to online video, with growth rates close
to 10% per year. This article reviews the latest findings concerning energy
consumption of online video from the system engineer's perspective, where the
system engineer is the designer and operator of a typical online video service.
We discuss all relevant energy sinks, highlight dependencies with
quality-of-service variables as well as video properties, review energy
consumption models for different devices from the literature, and aggregate
these existing models into a global model for the overall energy consumption of
a generic online video service. Analyzing this model and its implications, we
find that end-user devices and video encoding have the largest potential for
energy savings. Finally, we provide an overview of recent advances in energy
efficiency improvement for video streaming and propose future research
directions for energy-efficient video streaming services.
