More than 200,000 Victorian homes and businesses had their power cut on Friday and Alcoa Australia's Portland Aluminium had to power down potlines during a hot, chaotic day.

Audrey Zibelman, chief executive of the Australian Energy Market Operator, said the chief cause of the outages - the worst in two years - was the failure of about 1800 MW of thermal generators in the extreme heat, including two units of EnergyAustralia's Yallourn brown coal power station and a unit at AGL Energy's Loy Yang A power station.