The high energy or exciter unit, is powered by the aircraft electrical system, either AC or DC. The electrical energy received is stored until it is dissipated as a high voltage, high amperage discharge at a predetermined value through the igniter plug. An ignition unit carries a joule rating. Joules are defined as power (Watt) multiplied by time (Seconds). This means one Joule equals one watt. The amount of Joules output greatly depends on engine requirements and may vary between different types of engines. A high Joule rating may be necessary for engine startup or relighting at high altitudes after a flameout occurred. Joule output can be mathematically found as well as why the ignition charge is lethal by the following equation. For example, a turbine engine ignition system is able to store 2.500 VDC with an ionizing voltage of 700 VDC at the igniter plug. The current in the system is 220 amps and the plug arcs in 40 millionths of a second. The Joule rating in this case would be (table 1.1) :
Watts 
= 
Volts * Amps 
Watts 
= 
700 * 220 
Watts 
= 
154.000 
Joules 
= 
Watts * Time 
Joules 
= 
154.000 * 0,000040 
Joules 
= 
6,16 
The Joule rating calculated above would be similar to those measured in business jet engines. Large turbofan engines, like the CFM56, have Joule ratings between 14,5 and 16 Joules. The ignition exciter is therefore capable of producing a 14,000 to 18,000 volt DC output at a rate of approximately 1 pulse per second.
