Mobile devices dominate a majority of our business and private life. The increasing number of apps and powerful hardware ensure high demand and a growing usage rate. Based on the device sizes, the space for hardware is limited and this fact impacts battery size and, therefore, battery life. Mobile phone vendors look for ways to increase battery life through hardware and software features. This paper analyses two popular interface technologies for mobile devices, REST and WebSocket in regards to their energy consumption. It is shown that the use of WebSockets consumes less energy than the use of REST. We identify as the reason the pattern of REST to connect and disconnect, which consumes more energy than a connection that stays open.