The inte­gra­tion of small-scale Renew­able Ener­gy Sources in the Smart Grid depends on sev­er­al chal­lenges that must be over­come. One of them is the pres­ence of devices with very dif­fer­ent char­ac­ter­is­tics present in the grid or how they can inter­act among them in terms of inter­op­er­abil­i­ty and data shar­ing. While this issue is usu­al­ly solved by imple­ment­ing a mid­dle­ware lay­er among the avail­able pieces of equip­ment in order to hide any hard­ware het­ero­gene­ity and offer the appli­ca­tion lay­er a col­lec­tion of homoge­nous resources to access low­er lev­els, the vari­ety and dif­fer­ences among them make the def­i­n­i­tion of what is need­ed in each par­tic­u­lar case chal­leng­ing. This man­u­script offers a descrip­tion of the most promi­nent mid­dle­ware archi­tec­tures for the Smart Grid and assess­es the func­tion­al­i­ties they have, con­sid­er­ing the per­for­mance and fea­tures expect­ed from them in the con­text of this appli­ca­tion domain.