  We define the energy of a perfectly isolated system at a given retarded time
as the suitable null limit of the quasilocal energy $E$. The result coincides
with the Bondi-Sachs mass. Our $E$ is the lapse-unity shift-zero boundary value
of the gravitational Hamiltonian appropriate for the partial system $\Sigma$
contained within a finite topologically spherical boundary $B = \partial
\Sigma$. Moreover, we show that with an arbitrary lapse and zero shift the same
null limit of the Hamiltonian defines a physically meaningful element in the
space dual to supertranslations. This result is specialized to yield an
expression for the full Bondi-Sachs four-momentum in terms of Hamiltonian
values.
