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New Delhi (CNN) -- Crews restored full power to northern India on Wednesday as officials continued to search for the cause of the country's worst blackout in at least a decade.
Half the country lost power Tuesday after three electricity grids failed, leaving an estimated 600 million people -- more than the population of the United States, Mexico and Canada combined -- sweating in the stifling summer heat.
The grids were meeting 100% of demand Wednesday morning, the Power Grid Corporation of India said on its website.
The blackout, the second in as many days, brought trains and traffic on roads to a standstill, while businesses closed and millions of residents sweltered without air conditioning in the summer heat.
Back-up generators kept airports and hospitals running, but the biggest disruption in India's energy network in years has focused attention firmly on the fragility of the system.
Officials say an earlier, smaller blackout on Monday could have been the result of greater energy demand from states, particularly agricultural ones, which had been using more than their allocated share. But the cause of Tuesday's larger blackout has yet to be determined.
Analysts said the successive failures exposed the government's struggle to upgrade the country's power grid to meet the demands of a growing population.
"The country has added 16% to its power grid over the last year, and this pace of growth is likely to continue over the next two or three years, so it's not as if the government isn't doing anything," said Arun Sundararajan, a professor at the Stern School of Business at New York University
"They have projected a 40 to 50% increase in the power capacity of the country over the next five years," he said. "It's just that at a transition time like this you are going to see situations where demand outstrips supply."
With about 1.2 billion people, India has the world's second-largest population, behind China.
During the blackout, at least 300 trains were held up in the affected regions, said Anil Kumar Saxena, a spokesman for Indian Railways.
New Delhi's metro system also suffered delays before power was restored, causing chaos for many travelers. Traffic signals also were out, resulting in major jams.
Frequent power outages are also wreaking havoc on businesses.
Nuts and bolts maker Harjit Singh is forced to cut production in half due to power failures in his factory, on the outskirts of New Delhi. He said he went six days straight without power.
Singh runs machines on a big diesel-guzzling generator in his plant, which supplies truck and tractor companies.
The fuel Singh uses to run his factory has led to a spike in production costs. The owner said he has had to lay off daily-waged laborers due to cash crunch.
Miners in the Burdwan District of West Bengal state were hit by the blackout, too.
The district's top administrator, O.S. Meena, told CNN that 150 coal miners were working underground when the outage struck, stopping lifts.
The authorities switched to emergency supplies to run the elevators, he said. "All are safe," Meena said about the miners.
Airports and hospitals, running on backup power, remained operational, but many businesses closed, said Jyoti Kamal, senior editor for CNN-IBN. The cause of the problem was the failure to generate sufficient power to keep pace with surging demand, he said.
Power is considered a luxury in much of India, where a third of households don't have enough to power even one light bulb, according to last year's census.
Energy shortages tend to be more common during the summer, when demand rises.
Some of this summer's increased demand has been caused by farmers using more energy for irrigation and other tasks, in part because rains during this year's monsoon season, which began June 1, are down by more than a fifth. People are also using air conditioning units more to cope with high humidity.
The monsoon rains, which last through September but would normally be at their heaviest in July and early August, not only provide rain for agriculture and hydroelectric power, but serve as a natural coolant, said CNN meteorologist Brandon Miller.
Humidity exceeding 80% makes the mid-90s Fahrenheit temperatures feel like more than 100 Fahrenheit. This makes it harder for buildings to cool at night, and harder for people to cool themselves through evaporation of perspiration, all of which lead to higher energy demands, Miller said.
Power Minister Sushilkumar Shinde, who has ordered an investigation into Monday's outage, said it had been a decade since an entire grid last failed in north India.
Prakash Javadekar, a member of India's main opposition Bharatiya Janata Party, criticized the government for what he said was "a huge failure of management in the power sector."
India -- the world's fourth-largest consumer of electricity -- relies on coal for much of its energy but also uses hydroelectric power, which has been affected by the diminished monsoon rains.
Observers say the crisis has exposed the need for India to update its infrastructure to meet its growing power needs.
"Economic growth is constrained by inadequate infrastructure," among other factors, the U.S. State Department's country report on India says.
"Foreign investment is particularly sought after in power generation," it adds, as well as areas including telecommunications, roads and mining.
CNN's Jethro Mullen, Mallika Kapur and Jennifer Delgado contributed to this report.