Historically, flat-plate photovoltaic modules have been given a "peak-watt" rating indicating the power generated under 1000 W/m(2) global irradiance at a standard temperature. However, questions have arisen regarding the direct-normal irradiance, ambient or cell temperature, and wind speed (when it is specified) that should be used for evaluating the performance of flat-plate and concentrator modules. By studying the conditions that are observed when the global irradiance on a 2-axis-tracked surface is 1000 W/m(2), our analysis provides an objective, quantitative basis for the choice of the "peak-watt" rating conditions for both types of collectors. These observed conditions are consistent with commonly used values of 850 W/m(2) for direct-normal irradiance and 20 degrees C for ambient temperature. Evidence is given that wind speed should be increased from the commonly used 1 m/s to a more frequently observed 4 m/s. (C) 2000 Elsevier Science B.V. All rights reserved.