Energy-saving retrofit cost-effectiveness ratio refers to the evaluation of the economy and feasibility of an energy-saving retrofit project by comparing the input costs and energy-saving benefits. It is derived by calculating the ratio between the investment cost of the energy-saving retrofit and the expected energy-saving benefits. Generally speaking, if the cost-effectiveness ratio of an energy-saving retrofit is greater than 1, i.e., the return on investment is higher than 1, it means that the energy-saving retrofit is economically feasible and the expected energy-saving benefits will be greater than the input costs, thus realizing both economic and environmental benefits. On the other hand, if the cost-effectiveness ratio of energy-saving retrofit is less than 1, i.e., the return on investment is lower than 1, it means that the expected energy-saving benefits cannot cover the required investment costs, and it is necessary to reevaluate the feasibility of the retrofit program or consider other energy-saving measures. Therefore, the energy-saving retrofit cost-effectiveness ratio is an important indicator for evaluating the economic benefits of energy-saving engineering projects.