San Diego Gas & Electric Co., a unit of Sempra Energy, agreed to purchase all 26 megawatts of power from NRG Energy Co.’s solar power project in San Diego County.

NRG Solar’s Borrego Springs Solar project is expected to be completed in mid-2012, San Diego-based SDG&E said in a statement. Financial terms of the agreement weren’t disclosed.

The 25-year contract is the fourth SDG&E has signed in the last nine months for solar power.

Construction on the plant will begin later this year, Lori Neuman, a spokeswoman for NRG Solar, said in an interview. Borrego Springs is part of a portfolio of US Solar Ventures LLC projects NRG Solar bought from ArcLight Capital Partners in June, Neuman said.

Power from the project won’t require new transmission, Jennifer Ramp, a spokeswoman for SDG&E, said in an interview.