A DoE investigation using factorial and response-surface designs to analyze a solar–pellet combisystem in Sweden to optimize the system based on energy cost was performed. The same approach was also used to examine collector output energy. Investigated parameters were: building heating load, hot tap water consumption, collector flow rate, tank size, collector area, and estimated wood pellet cost. Cost- and performance-based regression equations were derived for optimal collector area and tank size for a range of buildings, providing tools for individual building solar combisystem sizing and optimization. Tank set-point temperature and estimated future pellet price were subjected to sensitivity analysis, and the influence of solar collector parameters and tank insulation level on profitability was investigated. The results indicate that a larger than expected collector area would be profitable due to inflation and the future price of pellets, and that tank size is less important to system profitability. However, tank insulation and set-point temperature were highly significant.