Switchgrass (Panicum virgatum) serves as a model dedicated energy crop in the U.S.A. Miscanthus (Miscanthus x giganteus) has served a similar role in Europe. This study was conducted to determine the most economical species, harvest frequency, and carbon tax required for either of the two candidate feedstocks to be an economically viable alternative for cofiring with coal for electricity generation. Biomass yield and energy content data were obtained from a field experiment conducted near Stillwater, Oklahoma, U.S.A., in which both grasses were established in 2002. Plots were split to enable two harvest treatments (once and twice yr(-1)). The switchgrass variety 'Alamo', with a single annual post-senescence harvest, produced more biomass (15.87 Mg ha(-1) yr(-1)) than miscanthus (12.39 Mg ha(-1)yr(-1)) and more energy (249.6 million kJ ha(-1) yr(-1) versus 199.7 million kJ ha(-1)yr(-1) for miscanthus). For the average yields obtained, the estimated cost to produce and deliver biomass an average distance of 50 km was $43.9 Mg-1 for switchgrass and $51.7 Mg-1 for miscanthus. Given a delivered coal price of $39.76 Mg-1 and average energy content, a carbon tax of $7 Mg-1 CO2 would be required for switchgrass to be economically competitive. For the location and the environmental conditions that prevailed during the experiment, switchgrass with one harvest per year produced greater yields at a lower cost than miscanthus. In the absence of government intervention such as requiring biomass use or instituting a carbon tax, biomass is not an economically competitive feedstock for electricity generation in the region studied. (C) 2010 Elsevier Ltd. All rights reserved.