Unit commitment, including economic dispatch, is a key component of short term operation scheduling of an electric energy system. Common industry practice is based on the use of a 'priority list' for generation scheduling and a deterministic model for power/energy demand. The priority list specifies the next unit to be started or shutdown in response to an increase or decrease in load. A common problem in the use of priority lists is that the next unit is improperly sized to meet the actual change in load. The algorithm proposed here is more accurate than the priority list method , much faster than dynamic programming which can hardly be applied to systems of more 5 machines. For a system of 41 machines, the algorithm can determine schedules in 0.1 second which is test enough for on-line control. Furthermore the total generating cost is superior to that obtained by dynamic programming successive approximations.