  Quasilocal formulations of black hole are of immense importance since they
reveal the essential and minimal assumptions required for a consistent
description of black hole horizon, without relying on the asymptotic boundary
conditions on fields. Using the quasilocal formulation of Isolated Horizons, we
construct the Hamiltonian charges corresponding to local Lorentz
transformations on a spacetime admitting isolated horizon as an internal
boundary. From this construction, it arises quite generally that the
\emph{area} of the horizon of an isolated black hole is the Hamiltonian charge
for local Lorentz boost on the horizon. Using this argument further, it is
shown that, observers at a fixed proper distance $l_{0}$, very close to the
horizon, may define a notion of horizon energy given by $E=A/8\pi G l_{0}$, the
surface gravity is given by $\kappa=1/l_{0}$, and consequently, the first law
can be written in the quasilocal setting as $\delta E=(\kappa/8\pi G)\delta
A$..
