What size cold water cistern and hot water cylinder do I need to supply a low-pressure mixer shower?

The minimum size cold water tank (cistern) we recommend for one shower unit is 115 litres (25 gallons). Hot water storage will depend on a number of factors such as, time in the shower, flow rate of the shower and the stored water temperature and recovery time of the hot water cylinder. As a rule of thumb a low-pressure mixer shower will deliver a flow rate of 5l/m and will use around 3 litres of hot water a minute. Therefore, this will use up the contents of a domestic hot water storage cylinder containing 117 litres of which 93 litres is useable, in approximately 30 minutes. If a booster pump is fitted to improve shower performance then the hot water used can be in the region of 12 l/m, which means showering time will be reduced to approximately 8 minutes. The larger the hot water cylinder the better.