Fossil-fuel power plants feeding the Massachusetts electric grid must abide by government regulations to reduce their carbon footprint on- and/or off-site.  RECs (Renewable Energy Certificates) are instruments that can be purchased on the open market to offset greenhouse gases.  Solar panels on your property generate both electricity and the fact that this electricity is carbon-free (RECs).  When a power company buys RECs, they are buying the right to claim clean energy.  You still use the actual energy generated by your solar array (and sell any excess to CMLP via net metering (see here and here)), but the RECs certifying this as renewably-sourced energy are now owned by that power plant.  This regional cap-and-trade program provides financial incentives to expand solar while also mandating that power plants bring down their emissions over time.

You can watch an EPA video explaining RECs here.

Class I RECs are available for all renewable generation built in New England after 1997.  Previous state programs minted SRECs (Solar RECs) via the SREC I (solar arrays that began operating between Jan 2010 and Apr 2014) and SREC II (solar arrays that began operating between Apr 2014 and Nov 2018) Programs.  RECs generated by solar arrays in the MLP Solar Program (solar systems turned on between Jan 2019 and Jul 2022) are owned by CMLP and retired to reduce the carbon footprint of Concord's electricity (see more here).

Currently, all solar projects built in Concord must include a production meter socket so that CMLP can potentially offer REC programs in the future.  Solar installers will need to put customer-owned equipment in that socket - either a production meter (enabling the property owner to produce Class 1 RECs) or jumpers (meaning the owner cannot produce Class 1 RECs).

Each MWh (megawatt-hour) of renewable power produced generates one REC.  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.  You may choose to sell or to retire your RECs.  

Sell:

SRECs are valued in the hundreds of dollars and Class I RECs sell for tens of dollars each on the open market.  Most solar owners use a REC aggregator who decides when to sell the RECs 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.

Retire:

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 retain the rights to claim that your solar array produces 100% carbon-free energy.