It depends on the manufacturer and their specifications. The energy level can vary from one type to another and the goal is to obtain 'equivalent energy'. The idea is to account for transthoracic impedance (amount of resistance) and most automated, semi-automated and manual defibrillators account for this as it senses the distance and resistance between the two pads.
For example, in a Physio Control product, you might select 360 joules but you don’t actually know the amount of energy delivered due to a number of variables so it is 360 joules of ‘equivalent energy’. As it was already indicated, you can equal the level of joules through different means. (Joules (Energy) = Voltage X Current X Time), any number of combinations depending on the voltage, the current and length of time it is expelled.
It is becoming more standard practice to defibrillate once at the highest energy setting and not ‘stacking shocks’ as the higher energy level has demonstrated a higher efficacy and often results in less defibrillations to overcome the fibrillation threshold.
Check this link, it simplifies it: http://www.resuscitationcentral.com/...asic-waveform/