In this paper, we evaluate the performance of low-power energy-harvesting wireless sensor node (SN) which has hybrid energy storage unit (ESU) for monitoring applications. This hybrid ESU has super capacitor (SC) as well as thin film battery (TFB) for energy storage. After the deployment of SN, temporal energy outage (TEO) occurs when the stored energy in ESU falls to a minimum level due to uncertain energy-harvesting process and leakage of energy, rendering it unable to transmit information to base station. In this paper, we estimate the duration to reach TEO state which is an important parameter for evaluating the performance of the hybrid ESU. The proposed model considers variation of leakage energy in SC. We also determine design perspectives of hybrid ESU for different range of performance parameters to maximize the performance criteria. Further, given total cost constraints, we determine the SC and TFB capacity that optimizes the performance criteria. The analytical results are validated by Monte Carlo simulation using MATLAB. The estimated TEO duration can ensure precautions in monitoring applications. KEYWORDS energy harvesting, energy outage, hybrid energy storage, super capacitor, thin film battery, wireless sensor node the available energy varies due to fluctuating environment conditions. Hence, SN is unable to harvest energy continuously from ambient sources due to uncertain EH process leading to energy scarcity in ESU. For these reasons, temporal energy outage (TEO) occurs when the EH SN runs out of energy for a while until it harvests energy again. In TEO condition, the SN becomes inactive and thereby fails to achieve its objective of sending information to BS. In control and monitor applications, BS receives data from SNs and applies the control. Thereafter, BS fails to apply appropriate control when TEO occurs in the system. Consequently, the loss of information significantly affects the quality of service (QoS) of the EH wireless sensor network. A prior assessment of TEO duration would help avert information loss. TEO duration refers to the interval of time the EH SN remains active using harvested energy as well as the stored ESU energy before going to TEO condition.The difficulties arise when we estimate TEO duration of EH SN with hybrid energy storage unit (ESU). Mostly, the hybrid ESU unit comprises of either a thin film battery (TFB) or a super capacitor (SC). 12-14 SC has more rechargeable cycle compared to TFB enabling SNs to sustain for a longer lifetime. However, SC leads to a faster battery drain out since it suffers from more energy leakage compared to TFB. Because of these challenges, hybrid ESU is a prime concern and is also used in the literature 15,16 for low-power energy-harvested wireless sensor network applications and in Thangaraj et al 17 for aircraft health monitoring applications. In order to estimate the TEO duration of a hybrid ESU of SN, a consistent performance criteria is required that can handle the uncertain harvesting process and volatility of TFB and SC energy storag...