In 1991, Dhar \cite{Dhar},\cite{DharRuelle} proves that the recurrent configurations of the sandpile automaton form an abelian group for the addition operator $\oplus$. In this article we study the identity element of this group for the sandpile automaton on rectangular grids of size $p \times q$. We prove that for $q \geq p(2+3 \sqrt 2)/2$, this identity is made of $3$ parts ($x < \frac{p(2+3\sqrt{2})}{4}$,$\frac{p(2+3\sqrt{2})}{4} < x < p - \frac{p(2+3\sqrt{2})}{4}$, $x > p - \frac{p(2+3\sqrt{2})}{4}$.) Extremal parts are symmetric whereas the central one has $2$ grains of sand on every vertex. We give a new method to compute the identity element of the group. This method is twice as fast experimentally as the other known methods.