Wind- and solar-power markets are booming. Clean Edge estimates wind grew to $17.9 billion and solar to $15.6 billion in 2006, up from $4 billion and $2.5 billion in 2000.

But even that white-hot growth isn't enough to slow greenhouse-gas emissions. After all, wind and solar power made up only 2.9 percent and 0.1 percent of the world's electricity in 2006 - not enough to puncture the soaring China- and India-fueled growth in demand for fossil fuels.

WinterGreen Research, a research firm in Lexington, Mass., thinks advanced batteries could make renewable energy a significant part of the world's energy mix. One of the biggest problems with wind and solar is they aren't steady and reliable. The sun doesn't always shine and the wind doesn't always blow. Those who need electricity on rainy or still days must rely on the electric grid - meaning that utilities would need to back up solar and wind power with other sources for those days, if solar or wind power were ubiquitous - or on backup power in the form of diesel generators or, more usually, batteries.

Current lead-acid batteries aren't good enough and cheap enough for most people, and for utilities, to rely on. They are physically too large to back up the whole grid, they take too long to charge and, at large sizes, batteries need to be maintained and replaced regularly. And more efficient lithium-ion batteries, the batteries in your laptop, are far too expensive.

That's where thin-film batteries, usually made of a lithium polymer, come in. While thin-film batteries in most products today offer more flexibility in shape, they have been just as costly as lithium-ion batteries. But WinterGreen Research says that's about to change.

Next-generation thin-film batteries will offer 40 times the efficiency of lead-acid batteries for the same price of $0.15 to $0.20 per watt-hour, according to the firm. They also charge in four minutes, not 30 to 45, and last 15 times as long as lead-acids, the company claims.

WinterGreen forecasts the batteries will enable wind and solar power to grow to a whopping 55.3 percent of the world's electricity in 2018, with wind making up 40.7 percent. "When you get 40X efficiency, then solar and wind become feasible," says Susan Eustis, president of WinterGreen Research,who plans to publish a book about this scenario with her daughter, Susie Eustis, in September.

How are these numbers possible? Instead of converting wind and solar power from DC to AC, to plug power back into the grid, Eustis envisions a future where wind and solar power could be stored on thin-film batteries as DC power, then that electricity would charge plug-in hybrids, as well as small DC appliances like coffee pots and irons. The idea is that homes would be connected to two electric systems - the current electric grid, and their own DC systems powered by solar or wind. "You stay on the grid, and start supplementing your current energy demand with DC capability," Eustis says. "The next coffee pot you buy would be DC."

But myriad risks could prevent the concept's realization. Vinod Khosla, founder of Khosla Ventures, says WinterGreen's projected cost for thin-film batteries is still too expensive. He says the cost needs to be $10 to $30 per megawatt-hour (far less than $0.01 per watt-hour) to make it viable for utilities to rely on wind and solar power for large portions of their electricity. He also says compressed-airstorage DESC, could become a competitor.

Of course, WinterGreen thinks the answer is single-home wind and solar projects, not large-scale grid systems. But the idea of a DC infrastructure seems improbable, industry insiders say. "Would you want to rewire your home?" Prudencio says. "Everyone's saying solar needs to be more plug-and-play for customers - easier to install, easier for customers to use. Having separate products flowing off of DC and AC doesn't seem to be compatible with making solar easier to install."

Still, Eustis argues that the scenario she and her daughter have outlined is not unrealistic. With the oil supply depleting rapidly and the growth of the middle class in emerging economies, oil prices are only going up, she says.

"It's just a question of demand," she says. "If gasoline goes to $5 to $8 a gallon, what are people going to do? This is a huge problem. People don't have a choice; they will have to do something."