Energy storage battery can be used as the standby power of thermal power plants because it has quick response in a scale of millisecond to balance instantaneous load fluctuation. However, the expensive cost of battery energy storage systems is one of negative factors for their commercial applications. Taking lithium ion battery, sodium-sulphur battery and vanadium redox flow battery as illustrations of standby power for thermal power plants, their technical and economic performances were studied by cost-benefit analysis. The results show that as the spinning reserve power sources, they can achieve remarkable economic benefits that their cost-recovering periods are far less than their life cycles, respectively, despite their high power costs and energy costs. The reserve capacity of battery energy storage systems can improve load rates of thermal power plants, decline gross coal consumption rates, decrease generating cost and the emission of carbon dioxide and pollutants, therefore have an effect of energy-saving and emission-reduction.