Green Banking Act This bill amends the Bank Holding Company Act of 1956 to allow bank holding companies to own up to 20% of companies engaged solely in producing or storing renewable energy power, such as wind and solar power. This allowance terminates 30 years after enactment of this bill unless Congress enacts a joint resolution of approval. This bill amends the Federal Housing Enterprises Financial Safety and Soundness Act of 1992 to require both the Federal National Mortgage Association (Fannie Mae) and the Federal Home Loan Mortgage Corporation (Freddie Mac) to develop underwriting guidelines for financing residential solar energy systems to facilitate the secondary market for mortgages on housing with such systems for low- and moderate-income households. The Federal Housing Finance Agency must identify ways to increase the financing for residential solar energy system property improvements in underserved markets.