According to a statement from EPA, eligibility requirements are as follows:
1) Product must be Energy Star certified consistent with applicable Energy Star Partner Commitments and the requirements set forth in the Version 5.0 Energy Star Program Requirements Eligibility Criteria for Windows. Product performance (U-factor and SHGC) must be certified by a certification body recognized by the U.S. Environmental Protection Agency (EPA).
2) Products must be independently certified to meet the North American Fenestration Standard/Specification (NAFS) with a Performance Grade ≥ 15. Acceptable certification bodies shall be accredited to operate in accordance with ISO/IEC Guide 65, by an accrediting body that is a signatory to the International Accreditation Forum Multilateral Recognition Agreement that operates in accordance with ISO/IEC 17011. NAFS certification does not require an EPA-recognized certification body.
3) Products must meet the applicable requirements shown in the table below:
Climate Zone U-factor SHGC
Northern ≤ 0.20 ≥ 0.20
North-Central ≤ 0.20 ≤ 0.40
South-Central ≤ 0.20 ≤ 0.25
South ≤ 0.20 ≤ 0.25