Empirical Analysis of the Relationship Between Solar Irradiance Variability and Grid-Connected Photovoltaic Power Output Stability in Lagos State

Grid-connected solar photovoltaic systems face stability challenges arising from the inherent variability of solar irradiance caused by cloud transients, aerosol loading, and diurnal cycles. This study empirically analyzes the statistical relationship between high-temporal-resolution solar irradiance variability and photovoltaic power output fluctuations at a 500 kWp grid-connected installation in Ikeja, Lagos State, South West Nigeria. Irradiance and power output data were recorded at one-second intervals using calibrated pyranometers and power quality analyzers over an eight-month period. Variability metrics including ramp rate, standard deviation of one-minute changes, and cloudiness index were computed from measured irradiance time series. Transfer function modeling was applied to characterize the dynamic relationship between irradiance input variability and inverter AC power output fluctuation after accounting for array electrical dynamics. Wavelet decomposition analysis was used to isolate the spectral contributions of different cloud transient timescales to power variability. Results indicate that irradiance ramp rates exceeding 30 W/m2/s occur on 14 percent of operational days, and these events generate power ramp rates exceeding grid interconnection agreement limits at the study site. The study fills a gap in empirical irradiance-power transfer function data for the humid tropical coastal climate of southwest Nigeria. Findings inform the sizing of battery energy storage systems required to smooth photovoltaic output variability to levels compatible with the Lagos 33 kV distribution network interconnection requirements. Keywords: solar irradiance variability, photovoltaic, grid stability, Lagos, ramp rate