Most typical household electricity bills in Northern Ireland will increase by £90 a year from July 1.
Significant inflation in world fuel costs were blamed by supplier Power NI for a rise which will affect hundreds of thousands of bill payers.
The announcement means an extra £1.73 a week in electricity costs for a typical household because of the near 18% increase.
Stephen McCully, managing director of Power NI, said: "We regret having to make the difficult decision to increase prices, especially since we cut prices by over 14% last October.
"Prices are roughly back to what they were in 2011, since the previous cut is unfortunately now cancelled out. The fact is that so much is dependent upon world fuel costs, which are outside our control and which have a volatile effect on the price we pay for wholesale electricity.
"There is never a good time to increase prices and it is something we do reluctantly."
He said the company was able to cut prices last year when many other UK electricity suppliers were raising theirs and it was always looking for opportunities to reduce prices again.
Power NI has 620,000 customers, 73% of the market, including a total of 36,000 business and 584,000 domestic customers. The increase will add £5.33 per week to the bill for a typical small business with charges of around £400 every three months.
The Northern Ireland Independent Retail Trade Association (NIIRTA) expressed disappointment at the 17.8% rise as its members were still struggling in tough economic times.
"Given that many members have expensive refrigeration costs, they are going to be particularly hard hit by this increase," chief executive Glyn Roberts said. "We are also concerned that the similar increase on domestic customers will also result in less disposable income and will have a negative impact on spending in our local retail sector."