The integration of small-scale renewable energy 1 sources in the smart grid depends on several challenges that must 2 be overcome. One of them is the presence of devices with very 3 different characteristics present in the grid or how they can inter-4 act among them in terms of interoperability and data sharing. 5 While this issue is usually solved by implementing a middle-6 ware layer among the available pieces of equipment in order to 7 hide any hardware heterogeneity and offer the application layer 8 a collection of homogenous resources to access lower levels, the 9 variety and differences among them make the definition of what 10 is needed in each particular case challenging. This paper offers 11 a description of the most prominent middleware architectures for 12 the smart grid and assesses the functionalities they have, consid-13 ering the performance and features expected from them in the 14 context of this application domain.