Two consecutive disconnection events led to the power outage across Spain and Portugal, according to the latter’s grid company REE.
In a briefing with journalists, an official from the outfit said it was possible the generation involved was solar but said more information is required to be definitive.
The Spanish PM said there was no problem with excess renewables on the system.
The grid managed to recover from the first event but the second was more damaging, ultimately leading to interconnectors with France being interrupted, according to REE.
Some reports suggest a larger loss of renewable power was experienced immediately after the interconnectors went down.
The loss of generation forced all other generators offline until the level of power hit zero, it is understood.
The findings are preliminary, officials stressed, and the PM said the investigation is ongoing.
Cyberattacks have been ruled out.