In recent years, a wide variety of photobioreactors have emerged for producing triacylglycerol (TAG) from microalgae as a feedstock for biodiesel with wildly varying forecasts of TAG and biomass production rates. This research describes the general photobioreactor concept and the dominating factors that determine microalgal production and the maximum attainable production rate of TAG. The information necessary for such an assessment is obtained from published articles and the results are compared to published photobioreactor production rates. Results indicate that at most 9% conversion of available solar energy to TAG is thermodynamically possible, resulting in an annual areal production rate of no more than 231 m(-2) a(-1). Given photosynthetic efficiencies and TAG contents of microalgae currently reported, production rates of no more than 15.81 m(-2) a(-1) should be anticipated. (C) 2009 Elsevier Ltd. All rights reserved.