This letter considers a point-To-point data transmission scheme over a fixed number of slots where the energy harvesting transmitter operates in harvest-use-store (HUS) mode and an imperfect supercapacitor with storage loss and energy leakage loss is used as an energy storage device. For these settings, the dynamic programming (DP) method is known to achieve the optimal throughput, but the ' curse of dimensionality' necessitates the search of schemes that allow for an improved performance-complexity trade-off. The work focuses on deriving a computationally efficient online transmission policy that achieves the optimal or near-optimal throughput. This letter presents a novel computationally efficient uniform thresolding based online transmission policy that achieves near-optimal throughput. The work also presents numerical results which demonstrate that the proposed policy with supercapacitor-based energy storage achieves better or the same performance as compared to the battery-based energy storage, thus allowing for a longer lifetime of sensor networks.