  Context. Among others, shocks are known to be accelerators of energetic
charged particles. However, many questions regarding the acceleration
efficiency and the required conditions are not fully understood. In particular,
the acceleration of electrons by shocks is often questioned. Aims. In this
study we determine the efficiency of interplanetary shocks for $<$100 keV
electrons, and for ions at $\sim$0.1 and $\sim$2 MeV energies, as measured by
the Solar Electron and Proton Telescope (SEPT) instruments aboard the twin
Solar Terrestrial Relations Observatory (STEREO) spacecraft. Methods. We employ
an online STEREO in situ shock catalog that lists all shocks observed between
2007 and mid 2014 (observed by STEREO A) and until end of 2013 (observed by
STEREO B). In total 475 shocks are listed. To determine the particle
acceleration efficiency of these shocks, we analyze the associated intensity
increases (shock spikes) during the shock crossings. For the near-relativistic
electrons, we take into account the issue of possible ion contamination in the
SEPT instrument. Results. The highest acceleration efficiency is found for low
energy ions (0.1 MeV), which show a shock-associated increase at 27% of all
shocks. The 2 MeV ions show an associated increase only during 5% of the shock
crossings. In the case of the electrons, the shocks are nearly ineffective.
Only five shock-associated electron increases were found, which correspond to
only 1% of all shock crossings.
