The present article analyses the effects caused by variable power. The analysis concerns a country-scale electricity system with a relatively high penetration of seasonally variable hydro power plants and wind farms in the total electricity generation in 2030. For this purpose, the Latvian electricity system was chosen as an appropriate case study, as around half of its electricity is already generated from hydro power and numerous wind farm installations are planned for 2030. Results indicate that in such systems high renewable power variations occur between seasons causing a high probability of power deficit in the winter and power surplus in the spring. Based on the results, the wind farms' influence on the power deficit and surplus occurrences are discussed in detail. Wind farm generation decreases the probability of the electricity system being in power deficit, but increases the probability of the system being in power surplus. In the latter situation, the maximum value of power surplus increases since it is enhanced by the wind farm generation. Probability equations to express these changes are provided. (C) 2012 Elsevier Ltd. All rights reserved.