Energy requirement consider the volume of water and temperature required. The bigger the collector, more hot water, but the decision must be economical as well, hence the calculation. To selected collector size must provide 90% of the hot water needs in the summer. The hot water consumption is lower in the summer therefore, using 90% for the summer provides accurate calculation for the whole year.
Solar collectors come in standard sizes of 10, 20, 22 or 30 evacuated tubes. Round the calculation result to the next size down to prevent too much heat in the summer.
Depending on your preference, either Metric or Imperial values may be used to calculate the number of tubes required. Please note: 1 kWh/m/day = 317.1 Btu/ft/day

(1) Water Volume = Actual volume of hot water used at the tap each day.
Most hot water-heaters have temperatures of 60C / 140F. For shower, a temperature between 42C / 45C (107F / 113F) is normal. Therefore, a 300 liters tank may draw 220 liters of hot water (at 60C / 140F) from the tank.

(2) Temperature Rise = target tap hot water temp - average mains cold water temp.
Target hot water temp is around 42C / 45C (107F / 113F)
Cold water temperature fluctuates by about 10C / 18F between winter and summer. The average cold water temperatures should be about 10C / 50F in winter and 20C / 68F in summer. This may vary depending on the region.