The maximum power produced by a solar cell is commonly determined by measuring the solar cell's J-V curve as a series of discrete J-V points, and finding the point with the highest power. Since the J-V point with the highest power might not coincide with the actual maximum-power point of the J-V curve, there is an associated uncertainty in the calculated maximum power. We derive an expression for this uncertainty, which depends only on the known quantities: temperature, short-circuit current and the sampling voltage interval; and on a rough estimate of the maximum-power-point voltage. One useful application of the expression is to determine the minimum sampling interval required to attain a desired accuracy. (C) 2002 Elsevier Science Ltd. All rights reserved.