  Shocks, modelled over a broad range of parameters, are used to construct a
new tool to deduce the mechanical energy and physical conditions from observed
atomic or molecular emission lines. We compute magnetised, molecular shock
models with velocities $V_s=5$-$80$ km s$^{-1}$, preshock proton densities
$n_{\rm H}=10^2$-$10^6$ cm$^{-3}$, weak or moderate magnetic field strengths,
and in the absence or presence of an external UV radiation field. We develop a
simple emission model of an ensemble of shocks for connecting any observed
emission lines to the mechanical energy and physical conditions of the system.
For this range of parameters we find the full diversity (C-, C$^*$-, CJ-, and
J-type) of magnetohydrodynamic shocks. H$_2$ and H are dominant coolants, with
up to 30% of the shock kinetic flux escaping in Ly$\alpha$ photons. The
reformation of molecules in the cooling tail means H$_2$ is even a good tracer
of dissociative shocks and shocks that were initially fully atomic. For each
shock model we provide integrated intensities of rovibrational lines of H$_2$,
CO, and CH$^+$, atomic H lines, and atomic fine-structure and metastable lines.
We demonstrate how to use these shock models to deduce the mechanical energy
and physical conditions of extragalactic environments. As a template example,
we interpret the CH$^+$(1-0) emission from the Eyelash starburst galaxy. A
mechanical energy injection rate of at least $10^{11}$ $L_\odot$ into molecular
shocks is required to reproduce the observed line. The low-velocity, externally
irradiated shocks are at least an order magnitude more efficient than the most
efficient shocks with no external irradiation, in terms of the total mechanical
energy required. We predict differences of more than 2 orders of magnitude in
intensities of the pure rotational lines of CO, Ly$\alpha$, metastable lines of
O, S$^+$, and N, between representative models.
