  We compare the thermal conductance $G_{\rm thermal}$ (at temperature $T$) and
the electrical shot noise power $P_{\rm shot}$ (at bias voltage $V\gg k_{\rm
B}T/e$) of Majorana fermions on the two-dimensional surface of a
three-dimensional topological superconductor. We present analytical and
numerical calculations to demonstrate that, for a local coupling between the
superconductor and metal contacts, $G_{\rm thermal}/P_{\rm shot}= {\cal L}T/eV$
(with ${\cal L}$ the Lorenz number). This relation is ensured by the
combination of electron-hole and time-reversal symmetries, irrespective of the
microscopics of the surface Hamiltonian, and provides for a purely electrical
way to detect the charge-neutral Majorana surface states. A surface of aspect
ratio $W/L\gg 1$ has the universal shot-noise power $P_{\rm shot}=(W/L)\times
(e^2/h)\times (eV/2\pi)$
