Light Bulb Energy Classes

The energy class of a device gives an idea of the energy consumption of the appliance. Class A is the most effective, the class G the least efficient. The design of the label of the energy classes is also standardized by European directives.

A light bulb from that provides 13 lm/W will be ranked higher than one that provides 12 lm/W. But how light bulbs are precisely categorized?

Justification of the parameters of the energy class

The life expectancy of an incandescent bulb is limited by the evaporation of the filament. Evaporation increases considerably with temperature. At equal temperature, more the filament is thick, more it will time to evaporate, so to break. So you can wear it at higher temperature. This is why stronger (thicker filament) bulbs are designed by the manufacturers to operate at higher temperatures, so have better performance.

Consider two bulbs of the same power, but different return. Energy classes are at the expense of the bulb more low yield (which, somehow, less “merit” to have a less good performance).

Case over damages: consider now two bulbs of the same performance, but of different power. The classes are at the expense of the most powerful bulb (which has, somehow, also less “merit” to have this performance since the filament could operate at higher temperature,…).

Energy classes are thus dependent on (lm/W) but also the power output (W).

Determination of the energy class

The electric power and luminous flux must be known. Let’s take an example: a 100W bulb and 1340 lm.


Lamp (Watts) electric power: P

Luminous flux (Lumens): F

Calculation to be done:

(1) if the following inequality is true, the bulb is a class A:

a incandescent bulb energy class 2

Here, we calculate: 0.24 x root (1340) + 0.0103 x 1340 = 22.6

However P = 100 W 100 is not less than 22.6. Inequality is not true. The bulb is not A class.

To be class A, should be that the bulb will produce 1340lm less 22.6W.

(2) as the bulb is not class A, an additional calculation needs to be done.

You ask Pr a reference power (purely artificial size):

a incandescent bulb energy class 3

For our example (100W, 1340lm):

PR = 0.88 x root (1340) + 0.049 x 1340 = 97.9

(3) calculation of P/Pr. It is the P/Pr value that determines the energy class (B to G).

Calculated P/Pr: P/Pr = 100/97.9 = 1.02

Compared with the following frames to determine the class (B to G).

a incandescent bulb energy class 4