What is the energy of a black hole? This might be a stupid question but given Einstein's general theory of relativity $E = m c^{2} $ what is the energy of a black hole? Isn't the mass of a black hole infinite? Wouldn't that be infinity multiplied by the speed of light squared?  
 A: Black holes are just objects which have very large mass (not infinity) and they are concentrated in very small region of space, Which is called Schwarzschild radius $$r=\frac{2GM}{c^2}$$
it means escape velocity from orbit equals the speed of light (thats why black hole is black,  light can't escape it) but black hole can also be considered as a blackbody, it not only absorbs heat but it also radiates it (Mostly in form of infra-red light and the energy of light(Photon) is calculated using this equation $E=hf$)
(if you want to calculate what energy will be released if all matter in black hole becomes energy you can use Einstein's equation $E=mc^2$) (if you want to calculate gravitational potential use this equation: $V=-\frac{GMm}{r}$)
Example: For example we want to calculate energy released if all matter in black hole in center of our galaxy became energy if we know that mass of that black hole is $8.62\times 10^{36}kg$. Using Einstein equation $E=mc^2$ we get that $E=7.747\times 10^{53}\quad Joules$.
More info about that black hole: Sagittarius_A*
So in short black holes don't have infinite energy.
More information:
http://en.wikipedia.org/wiki/Hawking_radiation
http://en.wikipedia.org/wiki/Black_body_radiation
