In this paper we analyze the eciency of therm energy obligations auction (descend- ing clock auction), implemented in the Colombian electricity market in 2008. Eciency is achieved when the winning bidder is the one with the lowest costs. It is important in the allocation of public services like electricity, since its absence can end in the allocation of electricity generation to agents with high costs, aecting not only the competitiveness of the market at scarcity periods, but also aecting consumers due to high prices resulting from inecient auctions. In this analysis the Nash-Bayesian equilibrium is veried tak- ing into account dierent valuation distributions. Additionally, according to approximate data valuations we calculate the optimal bids and compare them with the actual bids. As a result wend that for lower-cost bidders is optimal to over-bid and for average and high-cost bidders is optimal to shade their bids, all this in dierent magnitudes, which can lead to inecient allocations.