Energy Efficiency Existing Ship Index

The Energy Efficiency Existing Ship Index extends to the existing fleet a design-based efficiency requirement that had previously applied only to newly built ships. Introduced under MARPOL Annex VI as part of the IMO's push to cut carbon emissions, it requires an existing ship to demonstrate that its technical design meets a minimum energy-efficiency standard, expressed as the grams of carbon dioxide the ship would emit per unit of transport work under a defined reference condition. It is a one-off, ship-specific calculation that a vessel must satisfy, rather than a continuous operational rating.

Because the index is about the ship's inherent design efficiency, ships that do not meet the required value must be modified. The most common way to comply is Engine Power Limitation, where the maximum power of the main engine is capped, which lowers the ship's reference speed and therefore its calculated emissions. Alternatives include shaft power limitation, or physical improvements such as more efficient propellers, hull modifications, or energy-saving devices, and in some cases the use of lower-carbon fuels. The measure effectively brings older, less efficient ships closer to the standard expected of modern designs.

EEXI works hand in hand with the two other efficiency instruments. The EEDI applies the same design philosophy to new ships at the point of construction, setting progressively tighter standards for each new generation. The CII then measures how efficiently a ship is actually operated year by year. Together the design measures (EEDI and EEXI) and the operational measure (CII) form the core of the IMO's short-term greenhouse-gas measures. For owners, EEXI compliance has meant surveying the existing fleet and, in many cases, applying engine power limits, which reduces top speed but has limited effect on ships that already operate at economical speeds.