This is true. The OP asked how much electricity it takes to kill a person in the electric chair. It should be noted that several people have survived the electric chair…at least temporarily because they are usually re-electrocuted till they are dead.
In the case of Willie Francis (also mentioned in the link I provided above) the chair was wired incorrectly. While he was electrocuted it was not sufficient to kill him. His case went to the Supreme Court where those representing Francis suggested that electrocuting him again constituted cruel and unusual punishment which is prohibited by the 8[sup]th[/sup] Amendment. The court disagreed and Francis was successfully killed in the electric chair about a year after the first attempt. The even sadder part to this is the link above suggests that Francis was almost certainly innocent of the crime for which he was being put to death (I don’t know if that is true or not).
John Louis Evans was put to death by electric chair in 1983. However, it took three separate discharges for a total of 14 minutes before it was done.
Some who support the use of the electric chair say it is humane as the electricity almost instantly overloads the brain thus not allowing pain to be felt. Willie Francis described the experience as unbearably painful but electric chair supporters note that he was hooked-up improperly. The truth, of course, is only truly known by dead people killed in the chair. Whatever the case the chair is certainly gruesome to behold in use. Cooking flesh, bleeding, vomiting, bowel release and a bevy of other nastiness often accompany the process.