So, when you sign up for green energy, you’re really buying those renewable energy certificates.

“Short of having a solar array on your roof and keeping all those electrons to yourself, it’d be really hard to figure out what type of electrons you’re using if you didn’t have something like R.E.C.s to keep track of the math,” said John Rogers, a senior energy analyst at the Union of Concerned Scientists.

If you live in a state with a fully deregulated energy system (most Northeastern states, California and Texas) you can sign a supply deal with a renewable energy company. If you do, your provider will probably never mention the certificates behind the system. But, wherever the electrons really came from, all the energy you use will represent green power generation.

Those supply deals generally cost a bit more than a traditional utility bill. Prices vary, but the average household should expect to pay a few dollars more each month.

If you live in a state with a regulated energy system, you won’t be able to choose your electricity provider. In that case you can still buy renewable energy certificates online to help encourage green power, but that would be separate from your utility service and bill.

Either way, the World Resources Institute, the Environmental Protection Agency and the Union of Concerned Scientists have said that certificates are a legitimate way to foster the growth of the renewable energy industry.

Mr. Rogers, the energy analyst, called them “totally legit.”

Another thing you should keep in mind is that the market for certificates is unregulated. That means the government is not keeping an eye on producers to ensure that they deliver what they promise. There is an independent watchdog for green energy projects, though: Green-e.

Green-e does two important things: It verifies that the energy concerned actually does come from renewable sources, and it makes sure that the certificates are not double-counted. The E.P.A. recommends that consumers buy from companies with a Green-e certificate.