The Florida Solar Energy Center (FSEC) operates photovoltaic testing and certification program accredited by the American Association for Laboratory Accreditation (A2LA) and approved by the PowerMark Corporation (PMC). The photovoltaic (PV) module power rating certification is central to the FSEC program, as it is also required for the PV system design approval. A quality or accredited testing laboratory is required to provide its clientele with estimates of measurement uncertainties to assure testing quality and express the confidence of the reported results. These are also the specific requirements of the ISO standard 17025, A2LA accreditation and PMC approval. The FSEC PV testing laboratory has performed the detailed analysis of the uncertainty estimates in PV module power ratings and related parameters. This paper discusses the general procedure for estimating the measurement uncertainty and its application to develop formulation for the uncertainty in PV module power rating under outdoor testing, followed by an example of uncertainty estimates at the FSEC PV test laboratory.