Jerry is right the higher the power the more you feel it. I have been shocked at 35 joules it was not fun but was over quickly.
Sorry you haven't gotten a response, I know we have active members who could answer form their own experience.
From what I've read, the defibrillator action can range from uncomfortable to painful. I knon when I have had a defibrillator applied externally to kick AFib back to NSR, I have been put totally under an anesthetic - the normal procedure. That said, the discomfort (what the medical profession likely calls it) is far better than entering a dangerous fibrillation in the ventricle(s).