🔧 Electric Water Heater Design Calculations

A - Tank Capacity (Liter) Selection

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.

1️- Domestic Hot Water Demand Estimation

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          :    30–50 liters

Hand wash    :   3–5 liters

Kitchen sink  :   10–15 liters

Bath              :   80–120 liters

2️ - Capacity Estimation Formula

A simplified engineering estimation:

Vt=Vu * (Tu​−Tc​)​ / (Tt-Tc )

Vu=N*Ve

Where:

• N = Number of persons

• Ve = Each person use

• Vt = Required storage volume

• Vu​ = Total mixed hot water demand

• Tu​ = Desired outlet temperature (≈ 40–45°C)

• Tc = Cold water temperature

• Tt​ = Storage temperature (typically 60–75°C)

This formula accounts for mixing between stored hot water and incoming cold water.

3️ - Example – 3 Person Household

• 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 * (Tu​−Tc​)​ / (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 (Tu​−Tc​)​ / (Tt-Tc )= N*40*(40-20)/(70-20) 

Vt = N * 16

if N = 3   then Vt = 3*16=48 Liters

So:

👉 A 50 L  water heater would be appropriate.

4️ - Typical Tank Size Selection Guide

1 Person  : 30–50 L

2 Persons :  50–80 L

3–4 Persons  : 80–100 L

5+ Persons  : 100–150 L

👉 Download the Excel Calculation Sheet