The economics of two proposed solar thermal hybrid power generation systems (STHSs) have been evaluated. Each system consists of direct-steam-generation solar collectors, a steam accumulator and a gas turbine power generation system which uses steam as its working fluid. One (STHS-A) of the proposed systems emits CO2 generated by burning fuel, whereas the other (STHS-B) captures the CO2. Assuming that the systems are located in San Francisco, USA, where solar radiation energy is approximately the same as the global average, the levelized electricity costs (LECs) of the STHSs have been estimated considering future uncertainty of fuel cost and the capital cost of the solar collector. The LECs of combined cycle plants, which are considered to be one of the major thermal power generation systems in the near future, have also been estimated to evaluate the economics of the proposed systems. When fuel (methane) cost is 4.5 $/GJ, for example, the STHS-A has been estimated to be the most economical among the evaluated systems where the carbon tax is higher than a value in the range of 106-244 $/t-C, whereas the STHS-B is the most economical where the carbon tax is higher than a value between 368 and 475 $/t-C.