Smart Grid is a promising new paradigm for the next generation power grid that aims at a more efficient generation, transformation, distribution and consumption of electricity and at the integration of the new renewable energies in the equation. For its development, extraordinary two-way communication and great sensorization are required and at this point emerges the wireless sensor networks with energy harvesting technologies. In this article, we present an innovative technique for energy harvesting in the area of smart grid at the wireless sensor nodes through radio frequency (RF) electromagnetic waves: RF energy harvesting, making them energetically autonomous. Several specific use-cases and applications are proposed where nodes with RF energy harvesting have advantages respect batteries, wires or other methods. A general node hardware structure for their development and a special wireless communication protocol for energy optimization in this kind of nodes are proposed as well.