This report compares the economics of a solar rooftop mandate for new detached residential homes with a conventional home with gas heating and central air-conditioning. The home with rooftop solar is assumed to be an all-electric, net-zero-energy home that would generate as much solar electricity on its rooftop each year as it uses. The gas-heated home would be built to the same standard in all respects other than the gas space and water heating and gas cooking are replaced by efficient electric devices in the net-zero-energy home. The context is Montgomery County’s 2017 ambitious climate emergency resolution and a more recent statement by County Executive Marc Elrich about the possibility of a rooftop solar mandate. The County’s goal is to reduce greenhouse gas emissions by 80% by 2027 and achieve 100% elimination of emissions by 2035