The US Department of Energy states:
If retail electricity competition is allowed in your state, you may be able to purchase a green power product from an alternative electricity supplier. Some states have already implemented electricity competition. Check the Status of State Electric Industry Restructuring Activity map, prepared by the U.S. Energy Information Administration, to see if your state has passed an electricity competition law.
Even if your state is not implementing electricity market competition, you may still be able to purchase green power through your regulated utility. More than 860 regulated utilities spanning more than 40 states offer “green pricing” programs (see map of green pricing programs). The term green pricing refers to an optional utility service that allows customers to support a greater level of utility investment in renewable energy by paying a premium on their electric bill to cover any above-market costs of acquiring renewable energy resources.
Whether or not you have access to Green Power through your utility or a competitive electricity marketer, you can purchase renewable energy certificates (RECs). RECs (also known as green tags, green energy certificates, or tradable renewable certificates) represent the environmental attributes of power generated from renewable electric plants. A variety of organizations offer RECs separate from electricity service so that you do NOT need not switch from your current electricity supplier in order to purchase these certificates.
If you wish to purchase RECs in support of Green Power, see American Wind Program.