A resistor produces/dissipates heat when a voltage drop is applied. A human body has the property of electrical resistance.
How much heat dissipation would occur (or could theoretically occur) if Misaka used her ability to the max on a regular human of similar build to herself?
My attempt at setting up the math
For a compilation of information about Misaka's abilities, see the dedicated page on the Toaru wiki site.
Misaka's maximum electrical output is 1 billion volts (according to the toaru wiki, citing the Railgun manga's first chapter and the light novel's second volume's first chapter's second part)
I'm not aware of any stated limits on her ability to induce electrical currents, but according to the Toaru wiki page on the Railgun ability, Misaka's application of the ability uses approximately 2,250,000 amps, citing the Index light novel Volume 20 Chapter 3 Part 7. Quoting from the Toaru wiki page:
The third-ranked Level 5 is able to fire a metal projectile at three times the speed of sound by utilizing a great amount of electricity, supposedly somewhere in the region of 2,250,000 amperes, or about 1,000,000 volts, 1/10th of her power.
Note: I personally don't quite understand the logic/calculation with the amps, volts, and power in that quote. Maybe I'm missing some context.
According to Joule Heating, (quoting wikipedia,) "the power of heating generated by an electrical conductor equals the product of its resistance and the square of the current:
P = I^2 R."
For direct current (assuming that's the natural first choice for Misaka's abilities (though I don't doubt she could use alternating current)), one can use
P = I (V_A - V_B)and
P = V^2 / R.
The specific heat formula says
Q = m c delta_T, which can be rearranged to
delta_T/delta_t = P/(m c).
According to Which is the specific heat of human body? (A Q&A on our [chem.se] site), a good rough estimate for the specific heat of an average human body is
3.3 kJ/(kg K).
The resistance of human skin varies from person to person and fluctuates between different times of day. The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 ohms. Wet or broken skin may drop the body's resistance to 1,000 ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 ohms".
The Toaru wiki page on Misaka doesn't seem to have information on her body mass, but it does say that she is 161cm in height, and According to wikipedia (Human_body_weight#Ideal_body_weight):
The most common estimation of IBW is by the Devine formula [...]
Female ideal body weight = 45.5 kilograms (100 lb) + 0.9 kilograms (2.0 lb) × (height (cm) − 152)
I've provided a self-answer based on my own (limited) knowledge and understanding of physics and the Toaru universe. Please consider providing a better answer if you are equipped to do so!
Note: If you're wondering what practical purpose this question serves, it's as a precursor to "How does Misaka survive heat dissipation in her own body when using her abilities?"