The thermonuclear energy locked inside a white dwarf
is sufficient to blow the star apart.  In particular, white
dwarfs composed of carbon and oxygen, which are more common and
contain more thermonuclear energy than those composed of oxygen,
neon, and magnesium, can release up to 0.1% of the star's rest
mass energy as the carbon and oxygen are converted into
an unstable isotope of nickel.  The energy released in the
explosion goes into expanding the debris from the white dwarf
to velocities approaching 10% of the speed of light.  The power
we see radiated from a thermonuclear supernovae comes from the
decay of radioactive nickel to iron.  The light we see from
a thermonuclear supernovae is about 10% of the energy released
in the explosion, or 0.01% of the white dwarf's rest mass energy.