A few years ago, officials in India’s capital of New Delhi started noticing an unfamiliar spike in electricity usage on hot weekday afternoons and then again around 11 pm.

The reason: Families like Prerna and Nikhil Gulati. The couple received their first air conditioner as a wedding gift from 29-year-old Prerna’s parents a few years back.

They flip it on when her husband’s niece stops in after school, and at night when they go to bed. “We had never used an AC,” she says. “Our electricity bill has gone up, but it’s worth it if we can sleep.”

For the Gulatis, an air conditioner fulfilled a common middle-class aspiration. But the growing use of air conditioning among India’s 1.3 billion people is among its biggest energy problems. Demand also is rising across other developing countries, where climates tend to be hot and incomes and populations are growing. That puts India at the epicenter of a global conundrum: How to accommodate the escalating needs for electricity without setting off an unsustainable tidal wave of energy use?

A cadre of Indian bureaucrats is pursuing one of the most promising responses: more-efficient air conditioners. Energy efficiency is the unsexy side of the world’s energy debate. It gets little attention compared to efforts to install solar and wind farms. It is particularly critical in the developing world, where most of the energy growth over the next generation will take place. Developing countries, which consumed less than half the world’s energy in 2000, now account for 58%, according to the Paris-based International Energy Agency. By the year 2040 they will account for 67%, the IEA projects.