Falling wholesale electricity prices were the main reason for the 1% monthly fall in the Bord Gáis Energy Index in December.
Increased wind energy, which met a record 24% of the total electricity demand last month, along with a reduction in electricity demand due to the milder than normal winter weather, saw wholesale electricity prices falling by 5%.
The Bord Gáis Energy index stood at 146 in December, 2% lower than the same time the previous year.
The index also show that wholesale coal prices fell by 4% as fears of a disruption to coal supply from Columbia eased.
Events in Libya, Iraq and South Sudan continued to threaten global oil supplies, which helped prop up Brent crude oil prices last month. The oil element of the index was unchanged in December.
But the natural gas element of the survey rose by 2% with a short period of cold weather in early December pushing the monthly average price higher.