Energy consumption is a relevant matter in the design of IoT applications. Edge units—sensors and actuators—save energy by operating intermittently. When idle, they suspend their operation, losing the content of the onboard memory. Their internal state, needed to resume their work, is recorded on external storage: in the end, their internal operation is stateless. The backend infrastructure does not follow the same design principle: concentrators, routers, and servers are always-on devices that frustrate the energy-saving operation of edge devices. In this paper, we show how serverless functions, asynchronously invoked by the stateless edge devices, are an energy-saving option. We introduce a basic model for system operation and energy footprint evaluation. To demonstrate its soundness, we study a simple use case, from the design to a prototype.