Intensive research in photovoltaics has led to the appearance of emerging solar cells such as organic, perovskite, etc. As the solar-cell power conversion efficiency (PCE) is the main driver of the field, its accurate measurement is vitally necessary. However, the accurate PCE evaluation is not easy in a typical research laboratory and, because of enormous pressure of competition, the correctness and validity of the reported PCE values is a recognized issue (Energy Environ. Sci., v.5, 6513, 2012; Nat. Photonics, v.8 669, 2014). The reference-cell PCE measurement method is a workhorse in the community; however, this method is reliable only if the absorption spectra of the reference and test cells are coincident. In practice, this condition is generally not satisfied especially for emerging solar cells based on novel materials. Moreover, the accuracy in the PCE measured - a property crucial for a consistent experiment - is unavailable. Here, we present a laboratory measurement technique - the spectral technique - and demonstrate its applicability for organic, perovskite, and dye -sensitised solar cells. The spectral technique is based on the control of the solar simulator spectrum and allows reliable evaluation of the PCE values and their inaccuracies. We anticipate that the spectral technique will facilitate easier comparison of the results across the community and between various solar cell technologies contributing to credibility of photovoltaic research and development.