Domestic solar hot water is becoming a more common technology used specifically with the residential market of New Zealand. Recently domestic solar hot water systems have been economically identified as an option in commercial applications. Commercial building owners in the corporate world generally base decisions on economic reasons, therefore this research investigates the need to economically optimise the size of domestic solar hot water systems for eight separate commercial applications within Christchurch. All modelling has been completed using Consol’s heat pipe evacuator tube panels orientated North at an angle of 45 degrees. The TRNSYS simulation program is utilised to model the domestic solar hot water system in the eight commercial applications. Each commercial application has a unique domestic hot water load profile. The heat pipe evacuated tube is locally available from Consol New Zealand Limited. A common proportional relationship was utilised to define the relationship between the size of the storage tank and area of solar panels, which enables a range of domestic solar hot water system sizes to be used in the simulations. A proportional relationship is identified to economically optimise the size of commercial domestic solar hot water systems in Christchurch. This proportional relationship enables engineers and designers of commercial domestic hot water systems to confidently implement domestic solar hot water system designs. This provides an economically optimal solution in regards to the size of the solar component that should be installed during the rebuild of Christchurch.