Upgrading incandescent lighting to LED.

The nervous amongst you should look away now.

According to the Road Vehicles Lighting Regulations 1989, our indicators, and brake lights should be 15 to 36 watts. Given that LED lighting produces roughly 9 times the light for the same input compared to incandescent lighting, if you put a 15-watt bulb in your car, the light will be getting on for the equivalent of 150 watts output… Clearly the law hasn’t been amended to allow use of LED technology in older vehicles, so what should you do?

From my own meandering experience, the answer is to go down the sensible route by finding an LED bulb which gives you approximately the correct power output. Here is how to do it:

  1. Do not buy LED bulbs which say 5/21 watts. As stated above, this would create a nonsense 50/210 watts.
  2. Do not buy LED bulbs with “discrete” LEDs like this:

Each “high power” LED is about 20 milliwatts (12 x 0.02 = about 2 watts) – trust me, I have bought some in the past and despite what the vendor says, they are much dimmer than the bulbs you already have.

  • What you need are “Cree” LEDs, which are very high power, although less efficient than other types and get “warm” in use.

I have this type.

Now if you research online you will discover that these are rated at 60W, but as you know marketing is often misleading. These very high power “Cree” bulbs lose twice as much power to heat as the “high power” LEDs mentioned earlier so perhaps I should do the maths:

Incandescent bulb: 1-watt light output (will vary), so we will use the reference light output “1.” Most of the power input is lost as heat as the filament needs to get white hot before it gives off light.

“High power” LEDs: They do not need to get hot to give off light. Heat is a by-product as nothing is 100% efficient, so for the same power input, we get “9.”

As you up the power, LED gets less efficient as more energy is lost as heat, but even in the worst case you are getting much more light output than using incandescent bulbs.

 “Cree”: These get hotter than the “high power” LEDs (definitely warm in use), so the efficiency is reduced by about half in my humble opinion to “4.5.”

So, are you confused?

OK then let’s try and straighten things out.

The picture shows a “60-watt” bulb, but bearing in mind this is a marketer’s description, we should be taking it with a pinch of salt. What is being said is that if this were a regular LED bulb, you would get the equivalent of 60 watts light output – it is not, and as most of us do not own a light meter, they expect to get away with the claim.

The reality is nearer a 30-watt equivalent. Too powerful you say?

In reality, LEDs do not give out pure white light. You can get ones which do, but they are very expensive, and there is no need to splash the cash if you know what you are doing.

LEDs give off a slightly blueish “cool” 5000K or yellowish “warm” 3000K light frequency. This means you lose a bit going through your light lens. It is worth mentioning here that even if you use a red LED bulb behind a red lens, you will not get the same light output as using a white bulb.

  • Make sure you buy LED bulbs that use the reflector in the light unit, otherwise you will just get a pinpoint of very bright light.

Apart from the above type I am also using these and some 11W all of which I am happy with.

  • All other types IMHO are not bright enough.

Benefits of LED lighting:

  1. Instant on and off not the slow glow dim of incandescent.
  2. Lower power is both kinder to your old switches, and wiring.
  3. Much longer lasting should be fit and forget.
  4. Your battery will last that bit longer if you forget to turn your lights off, and overnight should be no problem if just sidelights (Remember to convert your number plate light, dashboard lights, clock etc.).

If like I have experienced, your MOT testing station says that you had a bulb out and that they replaced it, escort them to your vehicle and show them the LED bulb they did not replace. They get rather embarrassed.

Reference: http://www.legislation.gov.uk/uksi/1989/1796/made


  1. Paul,
    Have you had any issues with LED indicator bulbs not drawing sufficient current to operate the flasher unit? I have heard that they can give the same effect as having one bulb blown, i.e. they draw insufficient current to heat the bimetallic strip in the flasher unit to the point where it toggles, therefore the lights stay on permanently and don’t flash.

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