  Based on previously published multi-wavelength modelling of the GRB 170817A
jet afterglow, that includes information from the VLBI centroid motion, we
construct the posterior probability density distribution on the total energy in
the bipolar jets launched by the GW170817 merger remnant. By applying a new
numerical-relativity-informed fitting formula for the accretion disk mass, we
construct the posterior probability density distribution of the GW170817
remnant disk mass. By combining the two, we estimate the accretion-to-jet
energy conversion efficiency in this system, carefully accounting for
uncertainties. The accretion-to-jet energy conversion efficiency in GW170817 is
$\eta\sim 10^{-3}$ with an uncertainty of slightly less than two orders of
magnitude. This low efficiency is in good agreement with expectations from the
$\nu\bar\nu$ mechanism, which therefore cannot be excluded by this measurement
alone. Such an efficiency also agrees with that anticipated for the
Blandford-Znajek mechanism, provided that the magnetic field in the disk right
after the merger is predominantly toroidal (which is expected as a result of
the merger dynamics).
