  This paper extends the treatment of system equivalent flux density (SEFD) in
Sutinjo, A. T. et al. (2021) (Paper I) to interferometric phased array
telescopes. The objective is to develop an SEFD formula involving only the most
fundamental assumptions and one that is readily applicable to phased array
interferometer radio observations. Then, we aimed at comparing the resultant
SEFD expression against the often-used root-mean-square (RMS) SEFD
approximation, SEFDrmsI = (1/2)(SEFD^2_XX + SEFD^2_YY)^(1/2) to study the
inaccuracy of the SEFDrms.
  We take into account all mutual coupling and noise coupling within an array
environment (intra-array coupling). This intra-array noise coupling is included
in the SEFD expression through the realized noise resistance of the array,
which accounts for the system noise. No assumption is made regarding the
polarization (or lack thereof) of the sky nor the orthogonality of the antenna
elements. The fundamental noise assumption is that, in phasor representation,
the real and imaginary components of a given noise source are independent and
equally distributed (iid) with zero mean. Noise sources that are mutually
correlated and non-iid among themselves are allowed, provided the real and
imaginary components of each noise source are iid. The system noise is
uncorrelated between array entities separated by a baseline distance, which in
the case of the Murchison Widefield Array (MWA) is typically tens of
wavelengths or greater. By comparing the resulting SEFD formula to the
SEFD_I^rms approximation, we proved that SEFD_I^rms always underestimates the
SEFD, which leads to an overestimation of array sensitivity.
