RECs (Renewable Energy Certificates) are market instruments purchased by power plants to offset carbon emissions generated from their fossil fuel use. When they buy RECs, they are buying the right to claim clean energy. You still use the actual electricity generated by your solar panels (and sell any excess to CMLP via net metering), but the RECs certifying that this energy is renewable are now owned by that power plant. This regional cap-and-trade program provides financial incentives to build solar arrays while also mandating that power plants reduce their emissions over time. You can watch an EPA video explaining RECs here.
You may choose to sell or to retire your RECs.
Class I RECs are available for renewable generation built in New England after 1997. Each MWh (megawatt-hour) of renewable power produced generates one REC. Previous state programs minted SRECs (Solar RECs). SRECs are valued in the hundreds of dollars and Class I RECs sell for tens of dollars each. You can guestimate your REC production by multiplying your solar system size by 1.2. For example, a 5 kW(DC) system can be expected to produce about 6 RECs each year. Most solar owners use a REC aggregator who decides when to sell the RECs on the open market for the best price. You can shop around for an aggregator for this service – see the link in the middle of this page. Or you might want to be hands-on and do the trading yourself – see the broker / trader link at the bottom of that same page. Keep in mind questions such as the number of payments per year, the REC price, and the commission percentage.
On the other hand, you may elect to retire your RECs, meaning you will not gain any financial value from them. In doing so, you will be able to retain the rights to claim that your solar array produces 100% carbon-free energy.