French solar output surged to a record on Friday, exceeding grid operator forecasts and pushing power prices below zero.

Generation climbed to about 20 gigawatts, surpassing the previous high set in July, according to RTE data. Typically, predictions for strong solar output drive day-ahead prices negative as excess supply is priced in ahead of delivery. That didn’t happen on Thursday due to the unexpected nature of the record, and only intraday prices dropped below zero on Friday.