In the paper the author discussed issues concerning a coal output reserve in a group of collieries. The maximization of a coal output in a group of collieries determines the highest possible work productivity, which is one of key indicators affecting the economic efficacy in hard coal mining activity. The author focused on factors determining shearer’s productivity and its effective work time in a longwall. Technical and organisational determinants of work performance are taken into consideration to analyse technical and organisational solutions which make technological processes less labour-consuming. Additionally, the algorithm of the determination of potential coal output reserve in a group of longwalls was presented. The research analysis was focused on a selected group of longwalls of two collieries. The necessary input data derive from source materials of collieries and monitoring of longwalls’ work parameters. The undertaken study brought in the estimated changes of work performance in a selected group of collieries.