After calculating the heat loss (standing loss) of the electric water heater tank, the Energy Class is determined according to the EU Ecodesign / Energy Label standard:

EN 50440 / EU Regulation 812-2013
(for storage water heaters).

The classification is based on the standing heat loss S compared to a reference value calculated from tank volume.

1️- Standing Heat Loss (S)

This is the value you calculated from insulation design.

S  = Standing heat loss (W)

It represents the heat lost from the tank when the heater is not heating.

2 -  Reference Standing Loss

The regulation defines a reference standing loss depending on tank volume:

Sref = 16.66 + 8.33 × V ⁰·⁴

Where:

V = tank volume (Liters)

S ref = reference standing loss (W)

3️ - Energy Efficiency Index

The energy class is determined by comparing the actual loss S to the reference loss S ref.

Ratio =S / S ref​

4️ - Energy Class Limits

A+ S ≤ 0.5 Sref

A 0.5 Sref < S ≤ 0.75 Sref

B 0.75 Sref < S ≤ Sref

C Sref < S ≤ 1.25 Sref

D 1.25 Sref < S ≤ 1.5 Sref

E 1.5 Sref < S ≤ 2 Sref

F 2 Sref < S ≤ 2.5 Sref

G S > 2.5 Sref

5️ - Example

Tank: V = 50 L

Sref = 16.66 + 8.33 × 50 ⁰·⁴ = 56.5 W

Now assume your calculated heat loss  S = 38.6 W

Ratio = 38.6 /56.5 = 0.68

Class: 0.5 < 0.65 ≤ 0.75

✅ Energy Class A

6️ - Typical Standing Loss for Water Heaters

50 L 30–40 W

80 L 35–45 W

100 L 40–50 W

150 L 50–65 W

7️ - What Controls the Energy Class

Energy class mainly depends on:

• Insulation thickness
• Insulation thermal conductivity
• Tank surface area
• Thermal bridges (pipes, flange)

For example:

20 mm C–D

30 mm B–C

40 mm B

50 mm A