Abstract: To ensure the power supply reliability at the process of the new-type power system's establishment, a method of the quantitatively evaluation is proposed to characterize the endurance and degrees of the power system's generation maintenance. Firstly, "effective generation" is proposed to describe the generation's contribution in the reliable power supply by giving its definition, its engineering measurement and its effectiveness proved with real events. Secondly, based on the effective generation, the "generation maintenance space" and the "generation maintenance margin" are proposed to estimate the generators' maintenance limits and degrees to the power system operation, which are illustrated at real power-system operation. Lastly, based on the generation maintenance space and the generation maintenance margin, a method to calculate the minimum incremental demand of the regular power generation capacity is deduced and illustrated with real power system operation. The result shows that with the penetration of new energy generation the real generation maintenance space is decreasing step by step, which makes the generators' maintenance arrangements more difficult. Besides, to ensure both the power supply reliability and the generators' maintenance fulfillment, the increase of the reasonable effective generation is necessary.