This paper presents a unique modelling of harmonics and comparison between high irradiance and low irradiance in PV system connected to a grid. The signals used are actual tested waveforms in a distributed PV system. The percentage error has been calculated for the individual parameters in both cases. The comparative study using Chirp Z transform (CZT) with 7 term B Harris Window will enable to model harmonics using this algorithm in the future. The paper shows the performance of the method proposed in different measurement conditions.