With the rising penetration of photovoltaic (PV) plants on low voltage distribution systems, the generation of current harmonics as well as its impact on transformer operation is a current concern. The present research work develops a mathematical relationship of solar intensity (I(t)) with PV-inverter generated total harmonic distortion of current (THDi,inv.), and then uses IEEE recommendations to present the impact of THDi,inv on the life of a three-phase distribution transformer (TPDT). The validation of the presented model is done by real-time data monitoring from a 100 kWp solar rooftop photovoltaic (SRTPV) system, integrated with an 11 kV grid supply through a 63 kVA TPDT in the composite environment of north India. According to the results, decreasing I(t) values from 857 W/m2 to 35 W/m2 raises THDi,inv from 3.57% to 63.43%. It is also observed that the production of poor THDi,inv is high in winter season (daily average = 27.44%) in comparison to their values in summer season (daily average = 15.21%). For I(t) values less than 315 W/m2, the generation of large THDi,inv (above 15%) take place and it increases the loss of life (LoL) of TPDT by a factor of 6.0.