The IEC Standard (60)904-5:1993/2011 limited the calculation of Equivalent Cell Temperature (ECT) of PV devices from the VOC to irradiance above 0.2 Sun and a constant ideality factor. Increased error at low irradiance was attributed to a varying temperature coefficient (TC) of the VOC. In previous works, we showed that this was in fact due to rise of the ideality factor (near Open Circuit, OC) at low irradiance, and developed individual corrections for different monofacial c-Si PV modules allowing for accurate ECT evaluation down to ~0.03 Sun. In the present work, we compare the ECT to module back temperatures (Tback) measured with up to 16 sensors in two outdoor testing campaigns involving 6 monofacial, glass-backsheet and 5 bifacial, glass-glass c-Si modules made with PERC cells. We present a method to derive the irradiance-dependent ideality factor, nOC, allowing for accurate ECT evaluation and outdoor performance characterization also at the very low irradiance. Correction methods for the rated VOC and its TC are presented. We show the possible uses and challenges of the ECT–Tback plots over in-plane irradiance (T plots) and propose a simple effective irradiance in the bifacial PV case.