•A capacity design factor was defined to evaluate the system design and off-design operation strategy.•Key parameters that influenced on system performance under different capacity design scenario was quantitative analysised.•An universal system boundary of the ESR under different energy-matching scenarios was constructed. In this study, a capacity design factor was defined to evaluate the system design and off-design operation strategy, and the mathematical model was derived for the relationship between the load supply and the users' demands under different capacity design modes and operation strategies, and the influence of the key parameters (load demands, load rate, matching degree) on system integration and operation characteristics under different capacity design scenario was quantitative analysised. Furthermore, the impact of energy-matching degree and system efficiency on the energy saving rate (ESR) was discussed, and based on the user demands, an universal system boundary of the ESR was constructed. Finally, the energy saving potential in different scenarios was clarified, and a case study was conducted using a data centre as an example, and the optimal capacity design mode was demonstrated.