Low-power wide area networks (LPWANs) are emerging Internet of Things (IoT) technologies that support smart city development. An individual application inherits distinct features that demand different characteristics of LPWANs. For instance, various applications demand different data rates or tolerate different packet drop rates. As such, quantified performance of the LPWAN will help developers to make decisions on classes and types of LPWAN to be used. Previous works only provided qualitative comparisons on LPWANs. Thus, smart application developers are still facing the challenge of inappropriately adopting LPWAN technologies. Currently, the IEEE Standard Working Group P2668 is working on developing a standard to evaluate, grade, and rank the performance of IoT objects by using indicator values referred to as IoT Index (IDex). To achieve smart selection on LPWANs, the IEEE P2668 IDex Working Group has first focused on the evaluation of LPWANs and developed a novel LPWAN-I. The proposed LPWAN-I is a sub-index of the IDex that quantitatively evaluates the applicability of LPWANs and selects the most suitable LPWAN candidate for a specific application. This work is the first article to report IDex, which helps to deliver the best practice. Finally, the challenges and future development of LPWAN-I are also presented.