The olive and grape agro-industrial sectors have a major economic importance in Extremadura. Annual production of olive oil is more than 50 x 10(3) t, and of wine is more than 3 x 10(6) hectolitres. The large amounts of by-products are in most cases under-used, although they could be converted into a zero cost of the waste at the point of origin. In this context, the present work describes an estimate of plant size, and an economic analysis of grate tiring + steam turbine (GF/ST) and fluidized bed combustion + steam turbine (FBC/ST) waste-to-energy solutions using industrial olive and grape by-products in Extremadura. The fuel is dry olive husk waste (OH), olive mill wastewater (OMW), OH + OMW sludge, and grape waste from wineries, with total calculated specific costs of 3.28, 8.09, 2.67, and 2.05 epsilon GJ(-1) with respect to the lower heating value (LHV), respectively. The logistics component corresponding to trucking the biomass to the power production plant is that of greatest economic importance, even when the logistics strategy includes de-centralized drying plants. For real onsite availabilities of OH 21.084 x 10(3) t, OMW 37.483 x 10(3) t, olive sludge 87.462 x 10(3) t, and grape waste 89.486 x 10(3) t, the gross power is 19.13 MW for a GF/ST plant and 20.46 MW for an FBC/ST plant. The results are compared using standard economic indices-net present value (NPV), profitability index (PI), internal rate of return (IRR), and payback time (PBT). A sensitivity and risk analysis of the proposals showed the GF/ST option to be the better suited to the studied scenario, with better values for all the indicators. (C) 2007 Elsevier Ltd. All rights reserved.