Achelous is a model intended for forecasting mainly unregulated hydropower. Hydropower of this type is completely devoid of dams or has very limited possibilities in terms of storing water above the power plant and is therefore dependent on instantaneous access to the water that is supplied from the catchment area. In order to forecast unregulated hydropower, a number of weather parameters are used, of which precipitation in mm is the main one.

Achelous consists of a hydrological component that forecasts the access to water for the power plant, specified in mm of water for the catchment area, and a statistical component that does two things. On the one hand, it estimates the relationship between the volume of water and the utilised output, and on the other it estimates the offset in time between the supplied volume of water and the utilised output. The offset depends on the topography of the catchment area, the nature of the ground, the distance between the weather observation location and the power plant, systematic errors in the weather forecasts, etc.