This chart will help you estimate the hot water heater capacity one will need assuming a person uses 45 liters per shower or 65 liters on a regular sized tub.
Detailed breakdown is based on the following assumptions:
a. a person takes 10 mins to shower and consumes approx. 9 liters or 2.378 gallons per minute
b. a standard tub has 130 liters volume capacity
c. will use 50% of the hot and 50% of the cold water for both the tub and shower.
d. water pressure is around 1.5 to 2 bar
To give you an idea if you want to make a detailed computation, listed below are the water consumption based on Hansgrohe fittings at around 1.5 bar.
1. basin & bidet mixers : approx.7.0 liters per min.
2. exposed bath shower: approx. flow rate 22 l/min.
3. bath spout : approx. flow rate 25 l/min.
4. ordinary handshower mode is approx. 9.5 l/min without massage spray
5. rain overhead showers depends on the size, from a regular flow rate of 9.5 l/min to 11 l/min for those with limiters, it can go as high as 40 l/min for free flowing rain showers.
6. bathtubs sizes and volume capacity:
a. size 1.70m x 0.75m is approx. 130 to 200 liters
b. size 1.80m x 0.80m, 430 depth approx. 170 to 240 liters
c. size 1.90m x 0.90m, approx. 280
* please take note that Hansgrohe has a flow limiter that restricts to save water even if the pressure increases. So other brands will consume more capacity if pressure is higher.