The study considers a long-term assessment of annual carbon savings at a historic religious development. The research aim is to investigate and quantify the total monthly and yearly carbon savings from on-site energy production, usage, and banking rates from 2016 to 2021. This research critically assesses monthly and annual carbon savings in the historic development over an extended period. The building showcases Mid-Century Modern architecture and serves multiple purposes, including religious activities, communal offices, and educational and recreational uses. Completed in 1964, the development is in the northeastern region of the United States and is one of the state's historic buildings. The principal source of on-site energy generation is photovoltaic (PV) systems with a capacity exceeding 35,000 watts per year. The estimated U-values of the building components range from 1.8 to 3.0 W/m2K. The investigation includes actual data collection on on-site energy production, utilization, and savings over six years. This current study focuses on years with complete energy data for 12 months (i.e., 2017–2020) and performs data analysis for these years. The total annual carbon savings in the development, calculated using the kgCO2e/kWh benchmark for solar panels, varied from -0.46 to 3.64 tonnes. The highest quarterly equivalent carbon savings occur in each year's second and third quarters, with July recording the highest monthly carbon savings. The average annual carbon savings from 2017 to 2020 are estimated to be 1.03 tonnes, while the average yearly cost per kWh ($/kWh) is less than one US dollar. The research demonstrates that historic developments can be sustainable if appropriate retrofit strategies or interventions are implemented in their operations. The study suggests that historic buildings do not require complex interventions to be resilient, and their proper retrofitting can contribute to decreasing annual carbon emissions in buildings. The research outcomes can motivate owners of historic buildings to explore cost-effective strategies for enhancing their performance.