A method is proposed for determining the efficiencies of each face of a bi-facial photovoltaic module. The method employs one pair of bi-facial modules and one pair of mono-facial modules mounted horizontally at a height of approximately 3m above the ground. For each pair, one module is upward-facing, and the other downward-facing. The method ensures uniform illumination on all surfaces and provides an accurate measure of the ground-reflected radiation with the spectral difference between it and the sky radiation automatically taken into account. The relevant equations are derived and typical experimental results are presented as validation. It is cautioned that the meaning and usefulness of an STC rating for bi-facial modules is by no means clear.