Optimization of time-variable operation to maximize revenue through selling and purchasing electricity to/from the grid is presented for a thermal energy storage system. Time-variable electricity prices and electricity buy-back from the grid to re-charge the energy storage is considered by adding electric heaters to the CSPonD concept [Slocum et al., [15]]. System-level models are developed and optimization of the design and operation is performed. Three case studies are considered: i) time-variable operation without electrical heating under time-invariant electricity price; ii) operation under time-variant electricity price without electric heaters; iii) operation under time-variant electricity price allowing charging of the pond using the grid electricity. These demonstrate the effect of time-invariant versus time-of-use feed-in-tariff. Two hourly price profiles are considered, representing a moderately and highly fluctuating price profile respectively. Uncertainty of the cost function is indirectly addressed: the approach optimizes for the expected value of profit. The results show significant increase in the revenue when adding electric heaters. Under the moderately fluctuating electricity price, the use of heaters increases the revenue significantly, compared to the same case with no electric heaters considered. In the case of highly fluctuating electricity price, the use of heaters more than doubles the revenue. (C) 2013 Elsevier Ltd. All rights reserved.