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Why isn’t the output of headlight lamps simply given in watts?

By 2022-07-17November 30th, 2022Glossary

The reason is that the light output of a lamp is not generated by electrical power alone, but also by the temperature in the light bulb. The higher this is, the brighter the light becomes.

There are different ways to increase the temperature and thus the luminous flux. One possibility is to use a gas of higher pressure. Or a different material for the filament. Tungsten has a particularly high melting point, which is why it is used in halogen lamps. But even this metal can only be heated to a certain temperature by the electrical current. If you want to go higher, you need more watts of power input.

The other possibility for increasing the light output is to heat the filament even more. This is done by using a special reflector that focuses the light onto the filament. The radiation thus produced heats up the tungsten wire even more and makes it glow brighter.

This is why watts alone are not enough to describe the performance of a headlight lamp. The output in lumens and the luminance are much more relevant.

But that doesn’t make the choice of headlight any easier. After all, you can’t just look at the numbers to see how bright a lamp actually is. You have to trust the manufacturers’ descriptions – or test the lamps yourself.

Why isn’t the output of headlight lamps simply given in watts?

Watts alone are not enough to describe the performance of a headlight lamp. The output in lumens and the luminance are much more relevant.

Luminous flux and power

For household lamps, the world was simple for many decades. With light bulbs, also called bulbs because of their shape, it was the watt that counted in terms of brightness. A 60-watt bulb is more efficient than a 40-watt bulb, but when it comes to automotive lamps, especially those for halogen headlights, looking at the electrical output doesn’t help much.

what is watt

Let’s explain this with an example. Many headlight lamps have an output of 55 watts. So the H1, H3, H4 and H7. Do they all provide exactly the same amount of light? Not at all. For the light is largely the luminous flux in lumens decisive. And here there are clear differences in the truest sense of the word. The H1 produces 1550 lumens, the H3 a hundred less and the H4 even only 1000 in the low beam. The H7 produces 1500 lumens. The HB3 with 60 watts in its electrical data only insignificantly stronger supplies 1860 lumens and the high beam lamp H9 from 65 watts even 2100.

By the way: All these headlight lamps thus provide significantly more luminous flux than the household lamp with 60 watts, which only emits around 750 lumens.

Luminous flux is not the only criterion for the performance of a car lamp. Luminance has a considerable influence on what comes out of a headlight.

Another example: the H1, as mentioned above, produces 50 lumens more than the H7. Nevertheless, the latter is the more powerful lamp – because of its higher luminance.

So watts don’t have too much significance.

Frequently Asked Questions (FAQ)

The wattage of a bulb is not necessarily an indicator of how much light it will emit. Generally, a 60 watt incandescent bulb produces approximately 800 lumens of light. By comparison, an LED or CFL bulbs labelled as 60 watts might produce between 850 and 1200 lumens depending on the type and brand. It’s important to note that different types of bulbs may vary in their lumen output. As such, it’s best to consult product specifications for exact details when choosing a bulb for your lighting needs.

To identify the wattage of a light bulb, you need to look at its label. The wattage of a light bulb is usually printed on the base or side of the bulb in small letters. It should be easy to read with a magnifying glass; alternatively, you can hold it up to the light and read it by looking through the glass of the bulb. You may also be able to find the wattage information online if you have access to the manufacturer’s website or technical specifications. Once you know what type and size of the bulb you are working with, it will be easier to determine how many watts it uses. This is an important factor when choosing lights for your home as higher wattages result in brighter lighting but also increased energy usage. Knowing the wattage of a light bulb can also help you to choose the correct dimmer switch and other accessories needed to complete your lighting setup.
Standard light bulbs typically range from 25-watt to 150-watt. The wattage of a light bulb determines how bright the bulb will be and the amount of energy it consumes. A higher wattage means that the bulb is brighter and more energy efficient, but can also lead to increased energy costs if it is used frequently. It is important to select the correct wattage for your needs – too low, and you won’t get enough light, too high, and you could end up with an unnecessarily high electricity bill. Generally, 40-60 watts are good for general lighting uses in standard size rooms; while a 75-100 watt bulb may be required in larger spaces.
The answer to this question is yes, a 60 watt light bulb will be brighter than a 40 watt light bulb. This is because the higher wattage of the light bulb increases the amount of energy it produces and therefore makes it appear brighter.


David Muench (Carnes Mechanical)

David Muench (Carnes Mechanical)

Hey, I’m David. I’ve worked in a cars store for 6 years. I write reviews and guides, helping people to choose the most suitable technicals and best product for them. I’m happy to finally share my knowledge of the industry here, on CarnesMechanical.

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