How many middle school girls does it take to change incandescent light bulbs to fluorescents and scale clean energy to everyone in the US and beyond?

That was more or less the challenge put to some 250 young women by Dr. Kristina Johnson, CEO of the hydropower startup Enduring Energy, at a recent conference at the Princeton Plasma Physics Lab.

One student response: How many incandescent light bulbs does it take to die out before she should replace her entire home’s lighting system?

One might well ask, as the combination of world financial setbacks and energy crises affecting real lives and fuel bills brings the question home—quite literally. Global economies continue to reel from cataclysmic events, including last year’s off-shore explosion in the Gulf, the recent Japanese earthquake and tsunami setting off radioactive releases, and higher oil prices prompted partly by Middle East unrest.

Kristina Johnson, CEO, Enduring Energy

The road to clean energy is looking a little rocky, at best. The US Administration’s new energy security plan, released at the end of March, set a range of targets, including: cutting US oil imports by a third by 2025; expanding US oil and gas production, as well as biofuels markets; encouraging electric vehicles and energy efficiency; and, finally, creating markets for alternative, clean energy, with the goal of generating 80 percent of US electricity from clean sources by 2035.

To the student’s question, Dr. Johnson advised changing one bulb right away to see the results, and then waiting to replace the others as they die out—a thrifty compromise for belt-tightening families.

Until recently US Undersecretary of Energy, with broad responsibilities, from energy efficiency and renewables, to fossil and nuclear energy, to environmental and waste management, Dr. Johnson knows more than most about the sources and costs of energy—and cares more than a little about the intertwined future of young women, clean technology and the economy.

“Energy is a women’s and girls’ issue,” she announced to the young women convened at Princeton to learn about advancing their lives and careers through science, technology, engineering and math.

Dr. Johnson’s own story is a case in point. While still in high school, she was lured by the promise of ‘extra credit’ to develop an after-school project for a science fair, leading to a second project on holographic studies of fungus growth the following year. After the project won a first place in the international contest, Dr. Johnson headed for Stanford, where she earned her BS and eventually her doctorate in electrical engineering.

That led to a string of successes in academia, where she published numerous research papers, rising to Dean of Duke University’s engineering school and then Provost of Johns Hopkins University. Along the way, she also co-founded several start-up companies and earned over 120 US and foreign patents, including one that eventually led to the invention of the 3-D glasses used to view such movie hits as 2009’s Avatar, selling the company on International Women’s Day in 2007.

Her latest venture will generate and store electricity from hydropower.

Initiating an effort at the Department of Energy that convened a 2010 meeting with top women in energy, government and academia from around the world, Dr. Johnson is not just an inspirational role model but also an ardent advocate of women leading the way in clean energy.

Citing a study by the National Science Foundation, Dr. Johnson says “women will stay in a technology career when they’re allowed to affect a social issue,” so the challenge is to “mix humanities and social sciences and engineering to make a difference in the world.”

The opportunities in clean energy abound for women, she added, pointing out that engineers, economists and sociologists, among others, are all needed to bring about a shift to clean energy. And in the next decade or so, more trained professionals will retire—opening new paths for the next generation.

What’s more, she insists, to face increasingly complex world issues, including the economy, security, health and the environment, the US will need to develop the intellectual capital of currently underrepresented resources, including minorities and women.

Getting back to that light bulb, energy efficiency and rebuilding the country’s economy, she added: “Those energy jobs also pay well.”