The sensible heat storage in low-cost secondary fillers using a single tank thermocline system offers a cost-effective storage option for concentrating solar power (CSP) plants. A comprehensive numerical simulation of 125 MWht thermocline tank is performed by adopting a transient, two-dimensional, two-phase model to investigate the thermal performance of packed-bed thermocline thermal energy storage (PBTC-TES) system. The effect of relevant design and operating parameters on the performance of TES system are examined by analysing the thermocline expansion and local variation of salt and filler temperature. It is observed that the operating temperature difference (ΔT) has ample role on thermal performance of TES system as the efficiency is found to reduce by 12% with increase in ΔT from 50 K to 150 K. Discharging efficiency is estimated to be high at low operating temperature ranges and tend to decrease with increase in ΔT. Also with increase in power output, discharging efficiency tends to reduce slightly since it demands higher flow rates and operating temperature range. In case of inlet salt velocity, discharging efficiency drops by 3.5% when velocity is increased thrice the initial value. Further, a comparative study is performed to figure out the most dominant efficiency determining operating parameter. It is observed that relative to inlet salt velocity, operating temperature range seems to have more influence on the thermal performance of TES system. When the operating range is increased from 50 to 150 K, efficiency drops by about 10% whereas a reduction of only 2.5% is observed when power output is increased from 15 to 35 MWt for similar ΔT values. Hence, operating temperature range is identified as the dominant efficiency determining parameter of PBTC-TES system. © 2017 Elsevier Masson SAS