Load monitoring emerges in the smart grids context as an important part of the backbone of management systems. A variety of new products are offered by markets adding a certain level of intelligence in load monitoring systems, many of them focusing households to verify and in some cases regulate their own consumption. However, those systems even if they are affordable, some technical lacks are present limiting their utilization in research and development activities, e.g. the accuracy, completeness, consistency, and timeliness of information. Focusing in that, this paper proposes a sub-metering prototype system that fits well for the research and development activities for residential buildings in the smart grid context. Details of the system design and experimental results are provided which confirm the validity of the proposition.