Ignition energy can be determined by means of electric sparks produced by a transmission-line method. The energy released in the spark discharge is calculated from recordings of the voltage pulses incident onto and reflected by the spark gap. It is shown that the voltage across and the current through the spark gap are proportional to measurable distances in the graphs. By the introduction of a form factor, the spark energy can be calculated without detailed evaluation of the instantaneous spark power as function of time.