In annual terms, the country's power output was down by 24%, the office of statistics said in a statement, citing provisional data.
The April output of the country's hydropower plants went up 10% monthly and 43% yearly to 519 GWh.
The output of the thermal power plants in Slovenia totalled 244 GWh in April, up 2% on the year and down 22% month-on-month, while Slovenia's sole nuclear power plant Krsko was shut down for regular maintenance.
In April, Slovenia imported 922 GWh of electricity and exported 733 GWh. Exports declined 25% compared to the previous month, while imports increased 23%.
On an annual comparison basis, imports grew 25% and exports went down 14%.
Hydro energy had the largest share of 59.2% in Slovenia's total power production in April, followed by thermal with 27.8%, and solar and wind power with 13.0%.