An expansion of the Georgia Power and Southern Nuclear plant in Waynesboro may be delayed once more.

Georgia Power and Southern Nuclear are building two additional nuclear energy facilities, Plant Vogtle Unit 3 and Unit 4. The project, which started in 2013, is supposed to accommodate the state’s growing population

It has been riddled with delays. Now, construction quality issues and productivity problems may lead to another three-month delay, an independent monitor said Thursday.

This is the fourth schedule shift in the past six months. Unit 3 was supposed to be operational by 2016, and Unit 4 by 2017. Now, Don Grace, vice president of engineering for the Vogtle Monitoring Group, said Unit 3 will not likely be operational until the last quarter of 2022, and Unit 4 may not be ready by the first quarter of 2024, reflecting a four-to-five month shift.

Officials blame “productivity challenges” and the need for “additional time for testing and quality assurance” for the new delays.

The original price tag for the project was $14 billion, but it was last estimated to cost around $27 billion for all parties involved. Georgia Power said the updated schedule may result in a $1 billion increase. In October, the company said its projected cost alone would now be $9.2 billion.

The Georgia Public Service Commission is still trying to determine how much of the cost will be passed on to ratepayers. Under its current agreement with the Public Service Commission, Georgia Power will not recover about $694 million from consumers.

Officials said in October that every month of delay in the project equates to “an incrementally lower return, which translates to lower bill impacts during construction.”

Georgia Power customers already cover a share of construction costs. Rates have increased up to 3.4%. According to the U.S. Energy Information Administration (EIA), the average price for a residential customer in Georgia is 10.49 cents per kilowatt-hour. EIA data shows that the average commercial customer pays 9.96 cents per kilowatt-hour, and industrial customers pay an average of 5.55 cents per kilowatt-hour.

Company officials said the new units would contribute to their net-zero carbon emissions goals and create 800 permanent jobs. Georgia Power said customers are expected to save about $556 million in financing costs overall, in the long run, they said.