Capacitance

A capacitor stores charge (and energy) on two plates separated by an insulator.

Capacitance and energy

$$C = \frac{Q}{V}, \quad E = \tfrac{1}{2}QV = \tfrac{1}{2}CV^2 = \frac{Q^2}{2C}$$ Capacitance (farads) is the charge stored per volt. Energy stored = area under a Q–V graph.