hit by 1 billion volts

Mikoto looked at the boy who lay atop the bridge. Mikoto knew very well what would
happen to a human who received an electrical current of one billion volts. That boy
would not stand back up. She knew that. She had done it. She knew that.
And yet...
An instant later, she saw a movement coming from that boy who should never have
stood up again.
Gritting his teeth, the boy mustered all his strength and stood back up.

- Toaru Majutsu no Index Volume 3, Chapter 3 Part 3

At that point, she could no longer control the lightning spear as it pierced the boy’s chest.
There was a tremendous roar.
But the boy did not die. He did not even collapse to the ground. He gathered all his
strength in his legs and continued to stand in her way despite being so battered.

- Toaru Majutsu no Index Volume 3, Chapter 3 Part 3

As if to threaten him, Mikoto fired another electrical spear.
But Kamijou did not clench his right hand. The spear struck him directly in the chest.
Even so, Kamijou did not fall.
Even after receiving an attack like that, he would not fall.
“I have killed over ten thousand people! There is no reason for a villain like that to live
in this world! Why are you standing up for a villain like that!?”
“You aren’t a villain,” Kamijou said.
Mikoto frowned doubtfully.
“Why am I still alive?”
“Eh?”
“You said these attacks had one billion volts. A normal human cannot survive voltages
that high. Don’t you find it odd? Or are you subconsciously holding back?”
“Holding back?” Mikoto had a bewildered look on her face. “Of course not. I was trying
to kill you. I knew you weren’t resisting...I knew you wouldn’t resist...and yet...!”
“And yet you couldn’t kill me.”
“...”
Mikoto fell silent.
He was right. Normally, a human would not survive voltages as high as one billion volts.

But there was an exception.
For instance, commercial stun guns had voltages of two or three hundred thousand volts,
but a human would not die from being hit by one. On the other hand, one-hundred-volt
household outlets could electrocute someone.
That was not caused by the voltage. It was due to the difference in amperage. The
amount of electrical power was equal to the voltage multiplied by the amperage, so one
would not be electrocuted even with extremely high voltages as long as the amperage
was low.
In other words, Mikoto’s lightning spears had extremely high voltage but extremely low
amperage.

- Toaru Majutsu no Index Volume 3, Chapter 3 Part 3