NSTAR Green

NSTAR has started allowing people to pre-enroll for their 'NSTAR Green' option, which will let people pay extra to get their power from a windfarm instead of NSTAR's standard sources.  I expect to enroll in this, but one thing I'm wondering and not entirely sure how to find out is: is NSTAR under a mandate to use a certain percentage of renewable power or something similar?  If they are, this could feel like them trying to get customers to foot the bill for something they're required to do anyway.  Does anyone know if they are or where I could find out?  I can try calling NSTAR, but I don't expect the phone droids to be able to tell me this.