Use of higher energy doses (120-150 J) may not offer additional benefits and higher success rate compared with low to moderate initial energy (100-120 J) of biphasic shock for atrial fibrillation cardioversion. In this prospective open-label randomized trial consecutive patients with AF and candidate to AF electrical cardioversion will be randomized to receive 100J or 120 J as initial dose of biphasic shock. Patients will be also randomized in each group to anterolateral or postero-anteriore placement of defibrillator electrodes.