Unit Commitment (UC) is a term used for the strategic choice whereby the available power plants have to be on-line every time. Most UC models described in the literature are specifically designed for the power utilities. They are typical short-term models for relatively small power-systems. Apart from practical use in the utilities themselves, UC is also implemented in the broader context of electricity-generation modelling. For these purposes, however, the power systems can be much larger and the time scale more extended. Since UC is only a minor part of these models, the calculation time dedicated to UC has to be limited, thereby possibly sacrificing somewhat on accuracy. Two methods are compared. Unit Decommitment (UD), which is considered completely accurate and Advanced Priority Listing, which is less accurate but also less complicated. Simulations demonstrate that UD is slightly more accurate (0.03-0.6%), but takes much more calculation time (5-10 times more) than Advanced Priority Listing. (C) 2003 Published by Elsevier Ltd.