In this paper we present a new population-level life-cycle cost (LCC) analytical framework to quantify the relative costs and benefits of deploying solar water heating (SWH) end-use technology at a community scale, in comparison to individual systems, under a wide range of conditions. A primary innovation is to provide a quantitative definition of scale parameter to distinguish between individual-and community-level projects, and to identify major aspects of system cost and performance that are impacted bya change in scale.We describe the structure of the population-level LCC model and the methodology of its sub-models as developed for a study of SWH in California. We evaluate multiple residential building types and climate zones in California to understand factors that may influence the relative performance of community-scale installations vs. individual households. We developed a new hot water demand model, as well as a new multiscale SWH cost and performance model.Our results show that there exists an optimalscale for the SWH systems. A scale of eight provides maximum LCC benefits (minimum LCC) for single-family detached, a scale of 16 or 32 for single-family attached, and 32 for multi-family households. Minimal backup water heater natural gas use occurs at scalesoftwo to four for single-family detached housing, four to eight for single-family attached, and eight to 16 for multi-family housing. We support the results with population-level illustrations of scaling and climate zone effects on the energyuse and scaling effects on installation cost, total installed cost, purchase price and LCC.