Calculate capacitor stored energy, charge, and capacitive reactance from capacitance and voltage.
Capacitive reactance (Xc) is the opposition a capacitor presents to alternating current. It decreases with higher frequency and larger capacitance: Xc = 1/(2πfC). At DC (0 Hz), reactance is infinite — the capacitor blocks DC.
Energy stored equals ½CV². A typical 1,000 µF electrolytic at 50 V stores 1.25 J. Supercapacitors (1 F at 2.7 V) store 3.6 J. Compare to a AA battery at roughly 14,000 J.