###### Abstract
It simplifies the charging process of the above-mentioned energy-limited devices and provision the possibility of battery-free devices, which helps in device miniaturization. Furthermore, it can be viewed as an alternative to the charging devices that are present in places that can pose risk to human life or are difficult to reach. In addition, as this technology offers numerous advantages and can be used in many potential applications, it has attracted many companies [11] to view each other's to provide efficient, cost-effective, reliable, safe, user-friendly energy charging stations, and practical harvesters. Several wireless energy harvesting techniques have been discussed in recent studies; such as near field inductive coupling, near-field energy harvesting, magnetic resonant coupling, and far-field energy harvesting using radio waves [12]. In particular, inductive techniques, where the transmitter and the receiver circuits have magnetic coupling between them, the energy can be transferred from the transmitter to the receiver using the electromagnetic coupling between the two circuits. Obviously, this type of circuits work for short distances, such as wireless mobile phone charging and many other applications that do not require long distance between the charging and the charged nodes. On the other hand, RF-based charging is used for longer distance, where either a directive or non-directive method can be used. More details about the types of wireless charging technologies is provided in the survey paper [13]. Our focus in this paper is the far-field RF wireless charging technique. In short, the process of RF-based energy charging and harvesting can be summarized in three steps. First, the charging station converts the DC energy into radio frequency waves to be transmitted. Second, the generated electromagnetic wave is propagated into the free space. Third, the energy harvesting circuit receives the emitted radio wave and converts it back to the DC electrical energy that can be used to charge a battery or operate battery-free devices. In order to address the above-mentioned needs, recently, several companies have developed many solutions. In particular, PowercastInc. [14], one of the pioneers in this field has come out with evaluation kits for wireless charging that in turn can be adopted by researchers to explore on this emerging technology. The paper contributions can be summarized as follow:  
* This paper continues and expands our previous work on WC implementation and experimentation [15] such that it provides technical and practical insights, experimental verification, and evaluation of this technology using off-the-shelf (commercially available) products, thus making the results of this paper easily repeatable by others and facilitating the adoption of this technology in real applications;
* Provide a theoretical study on how to estimate the received power in wireless charging taking into consideration the distance between the wireless charging station and the target node, thus identifying the proper wireless energy propagation model. Identifying the proper wireless channel model is important in order to better estimate the amount of the received energy thus estimating the charging time and duration. Furthermore, the attained experimental results are compared with the theoretical results to demonstrate a close match that exists between the two results;
* The provided theoretical and experimental results assist the researchers and technology adopters to understand the technology, identify its capabilities and limitations while considering it for various applications such as WSN and the IoT;
* The paper introduces a use case for WC in the context of WSN, where the idea of utilizing the CH as a charging node for an unsupervised clustered WSN is proposed. Such a proposition is important due to the CH central location among the other nodes and its prime role in communicating with the other nodes, thus making it the best candidate for providing WC functionality to the sensor nodes.