Selecting the correct storage volume is one of the most important design decisions in electric water heater engineering.
Improper sizing leads to:
Insufficient hot water supply
Excessive energy consumption
Increased standby heat losses
Reduced user comfort
Tank capacity must be determined based on hot water demand profile.
The required tank volume depends on:
Number of users
Type of usage (shower, sink, kitchen)
Peak demand time
Desired water temperature
Cold water inlet temperature
Typical hot water consumption per activity:
Shower     :  30â50 liters
Hand wash  :  3â5 liters
Kitchen sink :  10â15 liters
Bath       :  80â120 liters
A simplified engineering estimation:
Vt=Vu * (TuââTcâ)â / (Tt-Tc )
Vu=N*Ve
Where:
N = Number of persons
Ve = Each person use
Vt = Required storage volume
Vuâ = Total mixed hot water demand
Tuâ = Desired outlet temperature (â 40â45°C)
Tc = Cold water temperature
Ttâ = Storage temperature (typically 60â75°C)
This formula accounts for mixing between stored hot water and incoming cold water.
N = 3 persons
Each shower - Ve = 40 L
Total demand - Vu = 3*40 = 120 L mixed water
Cold water - Tc = 20°C
Tank temperature - Tt = 70°C
Desired use temperature - Tu= 40°C
Using the formula:
Vt=Vu * (TuââTcâ)â / (Tt-Tc )
Vtank Vt = 120 * ( 40-20 ) / ( 70 -20 ) = 48 Liters
very simple formal function of N if we assume other parameters as above .
Vt = N *Ve (TuââTcâ)â / (Tt-Tc )= N*40*(40-20)/(70-20)Â
Vt = N * 16
if N = 3 Â then Vt = 3*16=48 Liters
So:
đ A 50 LÂ water heater would be appropriate.
1 Person : 30â50 L
2 Persons :Â 50â80 L
3â4 Persons : 80â100 L
5+ Persons : 100â150 L
đ Download the Excel Calculation SheetÂ