  We first review the various definition of the total energy in the
gravitational system. The naive definition has some defects, and we review how
to modify the definition of the total energy. Then we explicitly demonstrate
how to calculate the total energy of the system. Our example is the total
energy of a black hole in the expanding closed de Sitter universe in (2+1)
dimension. In general, we find that the contribution to the total energy comes
only from the singularity. Then we can calculate the total energy by evaluating
the contribution around the singularity.
