The firm says the average annual saving will be about £50 per household.
The move has been made possible by a steady fall in the price of wholesale gas, the main fuel used in electricity generation.
According to Power NI, its business and farm customers will also see reductions of between 7% – 11%.
Power NI is the North’s largest electricity supplier, used by more than 500,000 homes.
Managing Director Stephen McCully said: “This is the third year in a row we have been able to cut or freeze prices.
“This means more money in the pockets of householders and with farms and businesses also seeing a similar reduction it is good news all round.”
Power NI is facing ever greater competition from rival suppliers.
Earlier this month, Open Electric announced a 9% cut.
In January, SSE Airtricity announced a cut of just 1 per cent.