Yes, it's only a high level observation, but one that seems to hold true for electrical storage so far, i.e where there is a closed system with an electrical in

Yes, it's only a high level observation, but one that seems to hold true for electrical storage so far, i.e where there is a closed system with an electrical input and output. For fusion there are also various direct energy conversion methods for electrical output, but not the reverse, i.e i don't think there is method for nucleosynthesis directly from charged particles. In a more general sense, sure, power vs energy relation doesn't hold, but when limiting the system to the requirements to be a self contained electrical storage component, this relationship seems to emerge.