The Care Quality Commission (CQC) found "recurrent" building issues at GOSH, including power failures, water leaks, and ventilation problems.
These ongoing maintenance failures have impacted patient safety and resulted in delayed or cancelled surgical procedures, the report found.
The power cut involved staff needing torches to navigate the theatre during spinal surgery, though lights directly over the patient remained on.
A spokeswoman for the hospital trust said the incident was caused by an electrical system power failure and that an external review had been commissioned.