Re-Thinking the Definition of Self-Sufficiency in Systems with Energy Storage

Self-sufficiency is an important metric for various energy concepts, as it reflects what share of the local consumption is covered by local generation. However, the equation commonly used in literature cannot be applied to systems with an energy storage that actively exchanges energy with the grid. With more and more systems incorporating storage units it is therefore necessary to re-think the mathematical definition of self-sufficiency. The present paper addresses this issue by proposing an alternative equation that captures distinctive factors introduced by storage units: (i) Energy exported to the grid can originate from previously imported energy, (ii) initial and final storage energy in a given observation period can be different, (iii) the usage of storage systems entails energy losses. It is demonstrated that neglecting these factors leads to an over- or underestimation of self-sufficiency with values even reaching below 0 % or above 100 %. In contrast, the self-sufficiency calculated by the proposed definition considers the above mentioned factors and always stays within the defined range.