  The Einstein-Maxwell theory with negative cosmological constant in three
spacetime dimensions is considered. It is shown that the Smarr relation for the
electrically charged BTZ black hole emerges from two different approaches based
on the scaling symmetry of the asymptotic behaviour of the fields at infinity.
In the first approach, we prove that the conservation law associated to the
scale invariance of the action for a class of stationary and circularly
symmetric configurations, allows to obtain the Smarr formula as long as a
special set of holographic boundary conditions is satisfied. This particular
set is singled out making the integrability conditions for the energy
compatible with the scale invariance of the reduced action. In the second
approach, it is explicitly shown that the Smarr formula is recovered through
the Euler theorem for homogeneous functions, provided the same set of
holographic boundary conditions is fulfilled.
