= Solution
\b[<Specific energy>] is usable energy divided by device mass, commonly $\mathrm{Wh\,kg^{-1}}$; \b[<specific power>] is deliverable power divided by mass, commonly $\mathrm{W\,kg^{-1}}$. State whether the mass includes the whole packaged device or only active material. Volumetric <energy density> uses volume instead.
A conventional <capacitor> separates charge across a dielectric and stores $U=CV^2/2$, enabling rapid charge/discharge but relatively little energy. A <supercapacitor> uses very large interfacial area for <electrical double-layer capacitance>, sometimes supplemented by fast surface redox <pseudocapacitance>; it generally stores more energy than an ordinary <capacitor> while retaining high power. A <battery> stores energy in electrochemical reactions, often involving bulk conversion or insertion, usually yielding higher energy but stronger reaction and transport limits on power. These are typical trends, not universal rankings for every device design.
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