Energy conservation is tightly linked to the survival and fitness of overwintering ectotherms. To survive winter, many ectotherms enter diapause, a genetically programmed state of metabolic suppression. Most ectotherms diapause individually in sheltered hibernacula, however some form aggregations with conspecifics during winter. Aggregations are often associated with thermal buffering and water conservation, but the effect of aggregations on metabolic rates and energy use remains largely underexplored. The convergent ladybird beetle (Hippodamia convergens) occurs across North America and overwinters in large aggregations, making it a great candidate for testing the effect of aggregation size on metabolic rates in overwintering ectotherms. We measured metabolic rates of beetle aggregations of 1, 10, 25, and 50 individuals using stop-flow respirometry across two ecologically relevant temperatures. We tracked locomotor activity across aggregation sizes and temperatures to assess the role of activity and temperature in driving group effects on metabolic rate. Metabolic rates decreased with increasing aggregation size at both temperatures, but the decrease was steeper at lower temperature. Activity decreased with aggregation size, but only at low temperatures. These results suggest that individuals within aggregations enter a deeper metabolically inactive state that single beetles cannot achieve. This energy saving strategy may result from the aggregations’ boosted aposematic signal and thermohygric buffering, while single beetles face higher risks of predation and abiotic stress. This novel strategy for energy conservation may be an additional selective advantage for the evolution of large overwintering aggregations.