The amount of the energy credit claimed by the taxpayer in a taxable year may not exceed the taxpayer's personal income tax liability for that year. To determine the amount allowable in a given taxable year, a taxpayer must reduce the $1000 maximum credit amount by the amount of any energy credit allowed to the taxpayer in any prior taxable years with respect to the same principal residence.
http://www.mass.gov/dor/businesses/help-and-resources/legal-library/regulations/62-00-income-tax/830...
http://www.mass.gov/dor/docs/dor/forms/inctax16/addl/sch-ec.pdf
