The paper presents approach for determination of wind and solar generation probability to back-up power supply of customers in distribution network.The developed approach is based on the analysis and usage of statistics of real generation from wind and solar resources connected to distribution network, as well as customers’ load diagrams. Proposed approach was implemented in case studies that use information on real distribution network design principles, customers’ load diagrams and statistics of distributed energy resources generation in Latvia. Case studies are based on previously performed calculations (case studies) presented by authors at PQ2012 conference in the paper “Assessment of Different Power Line Types` Life-time Costs in Distribution Network From Reliability Point of View”. Results obtained from the calculations give well-grounded base for distribution network exploiting engineers as well as individual customers when trying to find network’s optimal reliability level by replacing overhead lines by other types of power lines (as one of network reliability centered maintenance and reconstruction activities of DSO) and by using small wind power generators and photovoltaics (as activity of customers).