Rapidly decarbonizing the global energy system is critical for addressing climate change, but concerns about costs have been a barrier to implementation. Historically, most energy economy models have overestimated the future costs of renewable energy technologies and underestimated their deployment, thereby overestimating total energy transition costs. These issues have driven calls for alternative approaches and more reliable technology forecasting methods. We use an approach based on probabilistic cost forecasting methods to estimate future energy system costs in a variety of scenarios. Our findings suggest that compared to continuing with a fossil fuel-based system, a rapid green energy transition will likely result in net savings of many trillions of dollars – even without accounting for climate damages or co-benefits of climate policy.