To accurately assess the overall energy efficiency of cruise ships and propose optimization strategies, it is necessary to establish calculation formulas for evaluating the energy efficiency of various onboard systems, combined with actual ship data analysis to reveal the characteristics of energy consumption distribution. This paper first constructs a typical model of the energy-consuming systems on a cruise ship, including the propulsion system, electrical system, and thermal system. Then, based on the first law of thermodynamics, corresponding calculation formulas and methods for the energy consumption, work output, and energy loss of each system are established. A comprehensive voyage calculation and analysis for a medium-sized luxury cruise ship is conducted. The analysis of the results reveals that the energy consumption of equipment and systems related to passenger services accounts for a high proportion, approximately 50% of the ship's total energy consumption. This finding shows a significant discrepancy from the simplified approach used in the International Maritime Organization (IMO)'s Energy Efficiency Design Index (EEDI) calculation for luxury cruise ships, where the auxiliary engine power is treated as a fixed percentage (about 5%) of the main engine power. The results of this study provide a methodological foundation for assessing the overall energy efficiency of cruise ships, lay the groundwork for proposing future revisions to the IMO's energy efficiency assessment methodology for cruise ships, and offer insights for optimizing the energy efficiency design of new cruise ships and retrofitting existing ones.