Field I-V curve measurements are an important means of quality assurance. They provide a possibility to verify the actual module power on a representative sample of modules in the field. The result is “worth money”, so it is essential that the uncertainty is kept at a minimum. This requires good knowledge of PV measurement principles, even if measurement equipment is readily available. Operators have to be aware of the influence of environmental conditions during measurements on the result. We performed a detailed uncertainty analysis in order to improve our procedure and measurement equipment. With our procedure, which follows the principles of IEC 60904-1 and applies procedure 1 in IEC 60891, we achieve uncertainties between 2.2% and 5%. This requires traceable primary calibrated measurement equipment and a thorough determination of temperature coefficients and other needed parameters. The major contributions to the combined uncertainty of corrected power are discussed in this paper. As it is important that the performance of thin film technologies can be assessed, we included not only crystalline silicon technologies, but also CdTe and amorphous silicon technologies in our analysis.