The use of nanostructured metal oxides as active electrode components of electrochemical energy storage devices provide benefits both from capacitive effects and from the short path lengths for faradaic processes, both of which becoming increasingly important at nanoscale dimensions. In this work, we analyze the capacitive and faradaic contributions to energy storage for Fe3O4 at two different nanoscale sizes in neutral aqueous media. The understanding of the contributions of each one of the effects at different sizes for different materials may provide clues for the design of devices with both high power and high energy density.