A first order dynamic model was developed and validated via experimental data for a PVT retrofitted collector. The model utilizes the analytic solution of collector's energy balance equation, with the purpose to estimate the absorber mean temperature via an iterative process. All collector's crucial parameters like the overall heat loss coefficient and collector efficiency factor are estimated for every simulation time-step, while PVT's optical efficiency is calculated analytically via the principal laws of optics. The simulation results fount to be in good agreement with experimental data. In more details, the RMS deviation for collector's outlet temperature was estimated to 0.66% for stable weather conditions and to 4.22% for extremely transient conditions with sporadic showers. Also, the discrepancy for power generation fount to be 5.05% and 14.91% for stable and transient weather conditions respectively. Moreover, for unstable weather without rain but with abrupt changes on flow-rate, the model found at sufficient accuracy with RMS deviation of 2.06% for water outlet temperature and 4.15% for power generation. Finally, by considering the absorber's heat inertia on the collector energy balance equation, a positive contribution on model's accuracy can be obtained, contrary to the steady-state version.