Capacitors act as temporary batteries storing electric potential energy that could be released over a short time.

In small engines, a capacitor-discharge ignition system is used that consists of a charging unit to charge a capacitor completely. Upon receiving a trigger, the capacitor rapidly discharges the stored energy to an ignition coil that, in turn, fires the spark plug to start the engine.  

The amount of energy stored by a capacitor-discharge ignition system depends on the value of the applied voltage and the capacitance of the capacitor.

Consider a fully charged capacitor with a charge of 375 micro coulomb and capacitance of 1.5 micro farad. Here the charge and capacitance are known quantities, and the voltage and energy are unknown quantities.

Using the known quantities, the voltage supplied to the system could be calculated to be 250 volt.

Upon substitution, the voltage and capacitance values give the value of the electric potential energy saved in the capacitor to be 0.047 joules.