  The potential field source surface (PFSS) equations are commonly used to
model the coronal magnetic field of the Sun and other stars. As with any
computational model, solving equations using a numerical scheme introduces
errors due to discretisation. We present a set of tests for quantifying these
errors by taking advantage of analytic solutions to the PFSS equations when the
input field is proportional to a single spherical harmonic. From the spherical
harmonic solutions we derive analytic equations for magnetic field lines traced
through the three dimensional magnetic field solution. We propose these as a
set of standard analytic solutions that all PFSS solvers should be tested
against to quantify their inherent errors. We apply these tests to the pfsspy
software package, showing that it reproduces spherical harmonic solutions well
with a slight overestimation of the unsigned open magnetic flux. It is also
successful at reproducing analytic field line equations, with errors in field
line footpoints typically much less than one degree.
