With the emergence of new network paradigms, network operators are able to provide slices (multiple simultaneous logical networks) thanks to Network Function Virtualization (NFV) and Software-defined networking (SDN) technologies. Currently, network operators do not have the means to commit on energy consumption, although it is a growing request from their customers. Indeed, given that energy consumption does not appear in slice Service Level Agreements (SLA) nor in Virtual Network Function Descriptors (VNFD), slice automated management systems cannot take this criterion into account. This paper addresses this issue with two contributions. First, we extend VNFD to include energy consumption. Second, we propose an SLA template for slices which includes energy based commitments. This template is complemented by some metrics in order to detect any deviation. Both contributions are illustrated by a use case. Our aim is to allow the entity in charge of referencing and placement of the virtual network functions (VNFs) for network service-composition purpose, to take into account energy consumption in addition to other preexisting aspects. This will help the network provider to commit on energy consumption with his customers through enhanced SLAs.