Why electric lights have two ratings written on the package

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

10 August 2025

A reader who prefers anonymity wants to know why LED (light emitting diodes) lights have two power ratings indicated on the packaging. “Bwana Kihanya: Which of the two is correct?”.

It is interesting how fast things change: just 25 years ago (at the turn of the century/millennium), nearly all the electric lights in our shops were the hot-filament type. This is a simple design where the electric current is passed through a thin resistor wire to heat it up to more than 2,500 degrees celcius. At these high temperatures, the wire glows in almost white colour.

Unfortunately, these kinds of lights are extremely inefficient in producing light. Over 90 per cent of the energy they consume is lost as heat and only less that 10pc comes out as useful light. However, for more than 100 years (from the early 1880s when electric lighting was commercialised to the late 1980s), the hot filament was the only type available for domestic use. Consumers got used to relating the brightness of the light to its power consumption.

Thus, for moderate lighting, the 60W bulb was used. This was the most popular brightness. 40W was slightly dimmer while 80W, 100W and 120W were brighter. In the late 1980s and early 1990s, compact fluorescent lights (CFL) started coming into the market for domestic lighting.

CFLs were branded as “energy-saving” bulbs because they were a lot more efficient than the traditional hot-filament type. The CFL produced about five time the brightness for the same power consumed. Thus, a 12W CFL was as bright as a 60W filament bulb.

Since the public was already accustomed to using wattage as a measure of brightness, CFL manufacturers found it necessary to show the equivalent in the same units. This is how the practice of writing two power ratings started – the smaller number is the actual power consumption while the larger one is the equivalent “brightness” of a hot filament bulb.

CFLs have now been replaced by LEDs which are about twice as efficient (that is, 10 times as efficient as filament type). Thus, a 7W LED is as bright as a 14W CFL, which, in turn, is as bright as a 70W filament bulb. Such an LED light will have the number 7W and 70W written of the package.

I am happy to notice that manufacturers are gradually dropping this practice and now indicating the brightness in the correct units: lumens. I recently bout bought a 7W LED and it says it produces 560 lumens of light.