The country's northern power grid failed Monday, then again Tuesday along with the eastern and northeastern grids, representing a total of about 50,000 megawatts of electricity, Press Trust of India reports.
About 20 of India's 28 states were affected by Tuesday's blackout and as many as 600 million people. Monday's outage affected eight states and a total population of about 370 million.
While India continually faces power outages and electricity shortfalls, the latest episodes were the worst in more than a decade.
New Delhi's metro rail system, which carries about 2 million passengers daily, went out of service for the second day in a row Tuesday. Roads were snarled with traffic, amid darkened traffic signals.
The northern grid failed at 2:35 a.m. Monday. That afternoon, Indian Minister for Power and Energy Sushil Kumar Shinde said 60 percent of electricity supply had resumed, as power was drawn from Bhutan and eastern and western grids.
Shinde attributed Monday's blackout to northern states drawing more than their allocated power quota.
"The reason for the outage was due to some states taking more power than they ought to have, which causes the frequency rate of the grid to go up," Shinde told a news conference Monday. "The offending states will be severely penalized."
Shinde said he appointed a three-member panel to look into the blackout.
Tuesday's massive outage hit at 1 p.m. The National Load Dispatch Center, a unit of Power Grid Corp. of India Ltd., in a 5:30 p.m. update on its Web site Tuesday, said work was in progress to restore the grid.
India's demand for electricity has soared along with its growing economy but utilities haven't been able to meet the growing needs. The situation is further worsened by coal shortages and bureaucratic delays on new power plants.
"India's generation capacity has outstripped its transmission capacity," the Financial Times quoted Arvind Mahajan, head of energy at KPMG India, as saying.
A report from the Indian government shows that last year 289 million people -- about 25 percent of India's population -- had no access to electricity.
Last week, the average shortfall in electricity supply was 8 percent on a national level and up to 22 percent in some states, India Today magazine reports.
Navroz Dubash, an energy expert at the Center for Policy Research, an independent think tank in New Delhi, points to mismanagement in the electricity sector.
"The electric power sector in India is like a bucket with a great big hole in it," Dubash told National Public Radio. "And so we actually have a problem where nobody knows exactly who is using how much power in India."