We quantify electrochemical losses in perovskite solar cells (PSCs) based on methylammonium lead triiodide (MAPbI₃) films with impedance analysis. We focus on the characteristic signatures of impedance spectra taken from PSCs, in particular the negative capacitance hook widely observed in the low frequency regime. We elucidate the underlying physical origin for the negative capacitance by applying a generalized equivalent circuit model (ECM) for PSCs that accounts for fast electrical dynamics resulting in high frequency (HF) signatures due to electronic processes, and much slower electrochemical dynamics that result in low frequency (LF) signatures in the spectra. We observe relaxation times faster than 10⁻⁶ s in the HF regime that can be attributed to electrical dynamics, while relaxation times longer than 10⁻³ s in the LF regime that are consistent with electrochemical dynamics. The voltage-dependence and timescales of the electrochemical dynamics are consistent with MA⁺ and I⁻ migration in the MAPbI₃ absorber layer. At higher applied voltages, we observe a highly non-linear response from the PSC which is consistent with irreversible chemical changes in the MAPbI₃ absorber. We demonstrate how ECM modelling combined with the analysis of ECM fit quality is a useful approach for in situ monitoring and quantitative diagnosis of loss mechanisms in PSC.