Renewable energy advocates are providing consumers with ways to lend their financial support to the cause.

Renewable energy certificates, also called “green tags,” are being sold across the United States by several companies that produce alternative energy. Some of these companies own wind farms; others own a variety of renewable energy sources.

Customers of the Boone Electric Cooperative will soon be able to purchase renewable energy certificates for electricity from the Bluegrass Ridge wind farm in northwest Missouri.

The certificates come in 100 kilowatt-hour blocks, and cost $2 more per block than conventional electricity. Al Lynch, assistant manager of Boone Electric Cooperative, said most members use an average of 1,100 kilowatt-hours of electricity per month. Replacing all that electricity with wind power would cost an extra $22 per month, he said.

“The more people purchase green power, the less coal has to be burned,” Lynch said

The Columbia Water and Light Department plans to buy power from Bluegrass Ridge as it increases its mix of renewable energy, but it has no plans to sell the certificates.

Residents in Columbia — or elsewhere — can still purchase the energy certificates from numerous companies that specialize in renewable energy.

The Pennsylvania-based New Wind Energy, for example, owns 11 wind farms in six states. The company offers 100 kilowatt-hour blocks of energy for $2.50 per month.

Buying the certificate, however, does not necessarily mean the electricity generated by renewable sources will make its way to the purchasing home. It flows instead into the power grid, displacing the same amount of conventional energy.

Paul Copleman of New Wind Energy uses a giant bathtub as a metaphor for the nation’s electric grid to explain the process: The bathtub is filled with electricity from many different faucets, with each faucet representing a different source of power.

“Every time you turn on a light switch, you drain a little electricity from the tub,” he explains. “Although buying wind does not mean that electricity is being directed straight from the windmill to your home, it does mean that more electricity being poured into the grid comes from the emission-free wind farm, rather than a dirtier generation source.”

Once a certificate is purchased, New Wind Energy releases the corresponding amount of energy to the grid.

According to the American Wind Energy Association, there are benefits and drawbacks to purchasing the so-called “green tags.”

One benefit is the flexibility for consumers to support renewable technologies and reduce their impact on global warming — no matter where the customer lives.

“The main drawback is that the green tag source may be far from a given customer’s home, so the customer is not able to take advantage of some of its localized environmental benefits, such as reduced smog and acid rain pollutants,” the association wrote on its Web site.