26 US Code ยง 25D Residential energy efficient property US Law

In the case of any qualified fuel cell property expenditure, the credit allowed under subsection (a) (determined without regard to subsection (c)) for any taxable year shall not exceed $500 with respect to each half kilowatt of capacity of the qualified fuel cell property (as defined in section 48(c)(1)) to which such expenditure 
Source: www.law.cornell.edu

Residential Energy Efficiency Tax Credit Department of Energy

For purchases made in 2011 2016: Aggregate amount of credit is limited to $500. Taxpayer is ineligible for this tax credit if this credit has already been claimed by the taxpayer in an amount of $500 in any previous year. For purchases made in 2009 or 2010: Aggregate amount of credit for all technologies placed in service 
Source: energy.gov