By 7.30 pm, power had been restored to North East India, and large parts of Delhi. Officials expect full supply to be restored in another two to three hours.
Today's outage comes after the Northern supply grid crashed yesterday, leaving seven states and Delhi without electricity. 300 million people were affected then. Twice as many were hit today.
"This is the second day that something like this has happened. I've given instructions that whoever overdraws power will be punished," said Power Minister Sushil Kumar Shinde.
The country's southern and western grids were supplying power to help restore services, officials said.
The Delhi metro stopped running for over an hour, and some passengers were trapped inside till an emergency supply helped trains reach the nearest station. Traffic signals on the blink had the capital stuck in traffic jams. In West Bengal, 200 workers in four underground coal mines were trapped for hours after the elevators to bring them back up stopped working; they have now been rescued. 65 miners are still stuck in mines in Jharkhand. Hospitals in the capital were without any lights till an emergency power supply kicked in. (Another Blackout: Share pics, videos)
Yesterday, Power Minister Sushil Kumar Shinde confounded many when he co-opted the crisis for bragging rights, and declared that his team had been able to restore power to most areas within six hours, much faster, he said, than during the last major collapse in 2001. Today, there was no such glass-half-full analysis, though Mr Shinde has reportedly been picked for a promotion to Home Minister, according to sources, who point to a cabinet reshuffle in the next few days.
Today's crisis was allegedly triggered after three states - Haryana, Punjab and UP - drew much more than their assigned share of power. (Why UP is considered the culprit) The power regulatory body - Central Electricity Regulator Commission - has summoned officials from each of those states on July 14. The regulator has also warned that any state that jeopardises the safety of a grid will be cut off.
The states hit today were - Haryana, Punjab, Rajasthan, J&K, Bihar, Odisha, Jharkhand, West Bengal, UP, Himachal Pradesh, Uttarakhand, Delhi and the seven North Eastern states.
In Kolkata, the metro was not hit by the outage, but West Bengal Chief Minister Mamata Banerjee asked all government offices to shut early, and urged the private sector and schools to do the same to protect against commuter chaos in the evening.
A grid receives power from generation stations, and passes it onto load centres, which is where distribution companies pick up their share, and pass electricity onto consumers. At grids, there has to be a careful balance between the supply and the amount of power collected. A line can trip if more power is drawn than provisioned for. And a grid needs mechanisms to ensure that if one line trips, it doesn't have a cascading or domino effect on the others.