But once you own the certificate, how can you be sure that your money ends up supporting solar, wind or other renewable energy efforts?

The emergence of a renewable certificates market a decade ago has given rise to independent third-party auditors such as the Center for Resource Solutions in San Francisco. The organization's Green-e tags are accepted by state regulators and the U.S. Environmental Protection Agency as a surety that the renewable energy certificate is legitimate. The Green-e tag certifies that the certificate purchaser paid for a specific amount of energy from a real producer.

In October, the group plans to start certifying carbon offset programs as well, to ensure that well-meaning people aren't paying hucksters for meaningless certificates.

One successful program that lacks independent certification is N.C. GreenPower, but it is endorsed by the N.C. Utilities Commission, a state regulatory body.

There is resistance

Still, some refuse to support it.

"I have no confidence that their product is what they say it is because they have chosen not to subject themselves to a third-party independent review," said Stephen Smith, executive director of the Southern Alliance for Clean Energy. "Our organization made a conscious choice that we would no longer promote their product."

N.C. GreenPower has been reluctant to undergo certification because of privacy concerns: Getting certified would require releasing contributors' identities to other accredited renewable marketers if N.C. GreenPower were to fold, said Jeff Brooks, N.C. GreenPower's marketing and communications coordinator.

N.C. GreenPower is structured so that instead of buying certificates, supporters make a charitable contribution to the Raleigh organization and qualify for a tax deduction. Last year, nearly 12,000 participants contributed $685,376, up 25 percent from the year before. Their contributions subsidize 88 in-state alternative energy producers, mostly small solar-power generators.

"Ours is the only program that guarantees the energy is coming from North Carolina," Brooks said. "In many cases, you're buying from some nebulous program somewhere in the country that will never benefit the local economy."

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