This standard describes a method to calculate the power and the storage volume required for the dimensioning of domestic hot water systems (DHW). The applicability ranges from direct water heaters (no storage volume and a comparatively large effective heating power) to long term storage systems (e.g. seasonal storage with a comparatively small heating power and large storage volume). This standard is applicable to the following water storage systems ¾ð storage charging systems characterized by a minimised mixing zone, e.g. layer-charging storage tanks or storage tanks with external heat exchangers, and ¾ð hot water tanks and storage systems characterized by a distinct mixing zone, e.g. storage systems with internal heat exchangers, and for different uses. The scope of the second part is to standardise the methods for determining the energy need for domestic hot water. This standard covers the domestic hot water needs in buildings. The calculation of the energy needs for domestic hot water applies to a dwelling, a building or a zone of a building. This standard also provides energy needs for different application cases of DHW-systems in hourly, monthly, and seasonal time steps, based on national default values.