Black holes can generate energy more efficiently than our Sun.
The material that is nearest to the fringe of the event horizon orbits and moves more rapidly, it heats up to billions of degrees Farenheit, which has the ability to convert the mass around it into energy known as black body radiation.
To compare, nuclear fission converts about 0.7% of mass into energy whereas a black hole converts 10% of mass into energy. The difference is far too big.
The disc of a black hole becomes extremely hot and stars radiate light allowing us to observe it before it is swallowed up. By analysing the motion of the disc and using the enamated light from it scientists estimate the mass of a black hole.