"We are in the grip of a profound energy crisis, the seeds of which were planted many years ago," Ramaphosa said in his annual State of the Nation Address to parliament.
The president's comments refer in large part to the "State Capture" scheme during the term of his predecessor, Jacob Zuma, in which state-owned-utility Eskom's funds were looted by persons close to that leader.
Eskom is implementing the worst rolling blackouts on record that are expected to wipe as much as two percentage points off economic growth in Africa's most industrialized nation this year.
South Africa's manufacturing output fell 4.7% year on year in December after falling by a revised 1.8% in November, statistics agency data showed on Thursday.
Pressure has mounted on Ramaphosa to provide solutions as South African households and businesses face hours of daily outages.
Declaring a national state of disaster gives the government additional powers to respond to a crisis including by enabling emergency procurement procedures and new regulations to be enforced.
It used the same legislation during the COVID-19 pandemic.
Ramaphosa also said on Thursday he would appoint a minister of electricity within the presidency.