Dye-sensitized Solar Cells are generally known to possess a slow response to changes in photocurrent generation by incident light. Electrical power measurements of photovoltaic (PV) devices can only be accurately made if sufficient time is allowed for complete photocurrent generation. The generation time increases from typically a few milliseconds in the case of c-Si devices, over tens to hundreds of milliseconds for technologies like back contact and hetero-junction solar cells, to even longer times for dye-sensitized and some types of perovskite solar cells. In this work we propose a procedure for calibration of slow responding PV devices based on an accurate evaluation of their response time. Starting from a quantitative analysis of the photocurrent signal versus chopping frequency on the spectral responsivity set-up, the measurement of a dye-sensitized solar cell was performed at 1 Hz chopping frequency. Then current-voltage (I-V) measurements were performed at different sweep-times and directions, in order to determine the correct parameters for I-V characterization. Combining appropriate spectral responsivity (for determination and correction of spectral mismatch) and I-V measurements yielded a reliable calibration of the device including measurement uncertainty estimation. Based on this work criteria for a reliable calibration of slow responding PV devices are formulated, fulfilling all requirements specified in the standards (IEC 60904 series and IEC 60891).