t might be too early to say what the exact cause of India’s latest massive power outage is, but in its simplest form, it probably has something to do with supply and demand –- not enough of the former and too much of the latter.

The outage, which left more than 670 million of the country’s 1.2 billion people without power, snarled traffic, shut down electric trains and idled some businesses. Indian officials say they are rapidly restoring power, but it’s unclear how soon the situation will be back to normal.

“In India, the power [demand] so far outstrips the supply locally and building new infrastructure is a huge issue,” says Anjan Bose, an electrical engineering professor and power grid expert at Washington State University.

“The problem has gotten more acute in terms of building enough supply. The transmission has generally kept up, but the building of new power plants has not,” he says.

Regular localized outages, known as load shedding, are common throughout India, as power grid controllers are forced to make cuts to keep the system in balance. As a result, many businesses, hospitals and airports use generators to make up the temporary shortfalls.

Amin Massoud, a senior member of the Institute of Electrical and Electronics Engineers and professor of electrical engineering at the University of Minnesota, has studied India’s power system firsthand. “In a big city like Bangalore, you have a half-hour to 45 minutes of load shedding every day,” he says.

The problem comes even though 40 percent of residences in India have no electricity at all, according to the World Bank.