Energy efficiency applications have great importance for facilities that utilize large amount of electrical and heat energy. Cogeneration (so called Combined Heat and Power; abbreviated as CHP) plants with gas engines are capable of generating both electrical and heat energy simultaneously using a single fuel input. In recent years, the realization of license exemption for facilities willing to produce electricity just for their energy demands by ensuring the condition of 80% total efficiency, low carbon emission of systems contain gas engines, rapid operation for synchronization and shortness of payback periods make cogeneration and trigeneration (so called Combined Cooling, Heat and Power; abbreviated as CCHP) plants more popular. This paper (i) briefly reviews cogeneration and trigeneration plants and their advantages, (ii) presents a novel methodology to determine the optimal capacity ratings for the plants by using the energy consumption profile, (iii) illustrates the calculation procedures including economic profit, thermal efficiency, and electricity generation of the selected system, and (iv) suggests the optimal capacity, plant placement and configuration for a medium-scale hospital. The energy savings potential at the university hospital is estimated as 19.66% and 19.52% with the use of natural gas based cogeneration and trigeneration plant, respectively. (C) 2015 AIP Publishing LLC.