Ecological payback time was calculated for demolishing an existing commercial building with average energy performance and replacing it with an energy-efficient, prefabricated building. A life-cycle assessment was performed for a 5,000 ft 2 commercial building designed by Project Frog and prefabricated in San Francisco, California, and compared to the impacts of annual energy consumption and continued status quo operation of a comparable average commercial building. Scenarios were run both with and without rooftop solar panels intended to make the prefabricated building net zero energy. The analysis considers the materials and manufacturing, transportation, annual energy use of the new building, and disposal of the existing building, compared to continued annual energy use of the existing building. The carbon payback of a new building with no solar against operation of an existing commercial building was found to be roughly eleven years, and a building with enough rooftop solar to be net zero energy was roughly 6.5 years. The full EcoIndicator99 environmental impact payback for a new efficient building with no solar was found to be twenty years, and a solar net-zero building was roughly eleven years against operation of an existing commercial building.