The paper provides a through analysis of electromagnetic fields harvesting efficiency. We show that harvesting of energy only from ambient sources implies significant limitations in applications with strong QoS requirements, such as modern wireless body-area networks (WBANs). The joint usage of ambient and dedicated sources can enhance the capabilities of EH-supplied WBANs. We also discuss the influence of various additional parameters on overall system performance (e.g. carrier frequency, WBAN mounting way, user movement pattern etc.). The obtained results are based on both laboratory measurements and simulations.