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I have a bicycle with lights and a hub Dynamo. I am looking for generic answers that apply to more setups but if that's not possible, the bicycle I have is a Stevens Randonneur 2021.

My bicycle lights can be turned off using a switch on the back of the front light. I was wondering if there is any benefit to turning them off during daytime. Two possible ones that I could think of (but don't know if are actually true):

  1. More efficient riding, as no power is redirected to keep the lights on. Probably tiny, even if true.
  2. Less 'wear' on components. The light and other components maybe last longer if not turned on unnecessarily.

Are the above true? Are there any other benefits?

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    Loss of some visibility to others is almost always a negative when not running at least some lighting. So if you are riding on a controlled path with little traffic and no motor vehicle traffic, it may be warranted to run without lights. However if you are in the proximity of motor vehicles, a few watts is a small price to pay for what may be your life if hit by a driver who does not notice you.
    – Ted Hohl
    Oct 26, 2022 at 6:33
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    @TedHohl in sunlight typical dynamo lights offer almost nothing in the way of visibility - far less than a jersey that contrasts with the expected background. But if you pass into deep tree shade the rear light (and the front on a narrow road) suddenly become very helpful indeed. That's a common occurrence where I ride, and partly why I leave mine on.
    – Chris H
    Oct 26, 2022 at 8:42
  • @ChrisH - I wasn’t aware of the poor performance of a dynamo-powered light (I remember having one on my bike as a kid, but that was half a century ago). Today’s LED light options are vastly superior that what was available even 10-15 years ago. I run a front light on my bikes that is the size of a one-inch cube, and it lasts many hours on a charge. It is not as much a headlight as it is for visibility. It also takes up very little space on the bar so the cockpit stays lean. Similar (red) setup on the rear. Total weight is maybe 3 oz/80 g for both.
    – Ted Hohl
    Oct 26, 2022 at 15:57
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    @TedHohl I've never used anything but modern LEDs with my dynamo, but they're all built to illuminate the road efficiently, which isn't the same thing as being seen in high light levels. The rear is more useful, but still, if I want to be noticed from a long way off in daylight, I'll turn on a flashing backup that would be dangerously bright at night in that mode.
    – Chris H
    Oct 26, 2022 at 20:22

4 Answers 4

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Yes, there is an efficiency gain by turning the lights off.

Cyclingabout made a hub dynamo comparison based on an experiment by skjegg, where power consumption was measured for different dynamos attached to different lights.

To give you ballpark figures, let's use the example of the Busch & Muller IQ-X light. The first graphic shows pedaling drag (dashed line) and output electrical power (solid line) for when the light is on.

Pedalling drag and output power for different hub dynamos with B&M IQ-X connected

The second graphic shows pedaling drag with lights disconnected, which is similar to turning the light off.

Pedalling drag with light off

By having an hub dynamo installed, you constantly loose around 1.5 - 5 W (at speeds above 15 km/h - lights off scenario). However, depending on light and dynamo, by keeping the lights off you can save around 8 - 11 watts.

An important follow-up question would be: does it matter in practice?

According to bikecalculator, for a 100 kg rider + bike system, flat, no wind, one needs about 115 W to cruise at 25 km/h and 39 W to ride at 15 km/h. Let's see how faster one could ride if this extra drag were to be converted into forward movement. I will use data from the Shimano 3D32 (the one I see most often) and assume that the calculator does not take into account hub dynamos.

So:

  • 15 km/h (39 W):

    • +2.5 W (41.5 W total effort - hub dynamo & lights off) would equal 15.5 km/h, i.e. approx. 0.5 km/h faster;
    • +13.0 W (52 W total effort - hub dynamo & lights on) would equal 17.42 km/h, i.e. approx. 2.4 km/h faster.
  • 25 km/h (115 W):

    • +3.5 W (118.5 W total effort - hub dynamo & lights off) would equal 25.24 km/h, i.e. approx. 0.2 km/h faster;
    • +10.5 W (125.5 W total effort - hub dynamo & lights on) would equal 25.85 km/h, i.e. approx. 0.8 km/h faster.

Note that these values are for instant speed. The average speed difference would be smaller.

EDIT: I originally made a mistake on interpreting the graphic that skewed the results significantly to the draggier side, thanks @jpa.

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    Wow, those internal dynamo losses are much larger than I expected. Thanks for providing experimental measurements. You have my upvote :-)) Oct 26, 2022 at 10:43
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    +1 but note that the IQ-X is one of the most powerful dynamo lights around, so results for more common lights will be a bit less
    – Chris H
    Oct 26, 2022 at 12:35
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    The above graph would be summarized more fairly by saying that the power "losses" due to the light on are between 5W and 10W. Rounding up to 10W is on the generous side... Oct 26, 2022 at 21:44
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    Also note, that those drag numbers for "lights off" mode seem to be too large. Other tests seem to come in much lower, e.g., 0.25 W to 1.25 W for the SON dynamos tested at 10km/h-30km/h as referenced here cyclingabout.com/dynamo-hub-drag-lab-testing (I wanted to mention it for completeness so that people can check out the other studies and don't get a wrong impression about how bad a dynamo hub is compared to a regular hub!) Oct 26, 2022 at 21:51
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    @gerrit No, at 20 km/h 5.5W is lost to drag and 4.5W is available to the light. Otherwise your interpretation is correct.
    – calofr
    Oct 27, 2022 at 8:46
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In general I leave mine on all day (SP-PD8 and Axa luxx 70) but it does take a little extra pedalling effort. A few Watts is likely to be a couple of percent of your effort, but if you were into marginal gains you probably wouldn't be riding with a dynamo in the first place. I do save the effort in bright conditions when the lights are of very little help, if I remember, and especially uphill.

The main reason to turn the lights off in my case is that then I can charge my phone from the dynamo as my light has a 5V USB socket.

Inside the dynamo, nothing will wear faster for drawing a current and the electrical parts don't move against parts they touch. LED lifetimes are at least tens of thousands of hours. Judging by mine the plastic lens will degrade before then, from little scratches.

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Many answers already state that there can be efficiency gains, but: There can as well be efficiency loss from turning off the light. This depends on the circuitry involved.

For example, if you turn off only the front light (many rear lights can not be turned off individually), the voltage generated by the dynamo will be higher. Instead of the typical 6V (RMS), it may produce 15V. The rear light may have a protection device (TVS diodes), which clamps the voltage at 12V. Now, as soon as the voltage would exceed 12V, much current will flow through the TVS diodes. This is probably less power than what the front light would consume, but it may be more power.

There are also dynamos with integrated TVS diodes. These are known to make more drag when not connected than when shorted.


Years ago I have made experiments with a Shimano 3D32. Of couse I can not link references for these experiments.

This model generates a kind of sine wave when under normal load and speed. In an open circuit it adds a sharp spike in the middle of each half wave. It generates an RMS voltage of about 1V per km/h, so at a typical speed of 20km/h it generates 20V RMS, which is much more than the rated 6V. The peak voltage is about 4V per km/h, so 80V at 20km/h. If you touch the wires of the unloaded dynamo at this speed, it actually hurts a lot!

The Shimano 3D32 is able to deliver much current if it is allowed to reach this peak voltage. I connected a 24V TVS diode, and at 20 km/h it burned about 15W. Therefore: If you have this dynamo, a rear light with 24V TVS diodes, and turn off your front light, you should actually notice the efficiency loss.

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  • I have managed to burn the protection diodes in a halogen headlight when the bulb was broken and I did not notice it right away. It was a bit unexpected.
    – ojs
    Oct 27, 2022 at 20:39
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This depends on the electronic design of your light and dynamo but as of the time of writing it is rather inefficient and the drag is not non-existent.

A bicycle dynamo historically produced between 1.2 and 2.4 W so the lamp could not be more powerful. LEDs are typically at least ten times more efficient (6 W LED shines like 60 W incandescent bulb, bright enough as a primary beam of a car). It would kind of be logical to expect the modern dynamo LED light to draw 0.1 W at moderate brightness but nope, looks like the majority of the recent dynamo lights still draw up to 10 W of power instead. They should shine like 100 W light often seen as too bright for a living room, what are they doing with this power?

This is likely tied to dumb/naive solutions that are used to convert the varying voltage of dynamo into fixed voltage required for the LED. While the efficient converter can be easily built, a simplest naive stabilizer is highly inefficient and looks like exactly these are still mostly used.

Hence your dynamo light may be creating up to 10 W of drag resistance that depending on your fitness and overall situation may make it unreasonable to keep it always on.

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    Those 4.5 volt torches were also incredibly dim compared to even non-halogen dynamo lights. 1.125W LED would be unnoticeable during day, and maybe just barely enough for riding if there aren't other light sources around and you have enough time to adapt to dark.
    – ojs
    Oct 26, 2022 at 15:32
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    @cmaster-reinstatemonica no, the light would need to be designed to be capable of operating with variable voltage, which it should be. (Yes, many aren't, but that's just bad design.) Oct 26, 2022 at 20:54
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    This seems to be an answer based on what could be achieved in theory, rather than what is achieved in practice.
    – gerrit
    Oct 27, 2022 at 8:08
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    Looks like even high end dynamo lights are not using the modern power converters and draw about the same power as the older halogen lights, relying on the stabilizers of 1970s design for mitigating the wide voltage range of dynamo. Shame, they probably do not see 10 W as worth considering. I have adjusted the answer. May be a good idea for a startup.
    – nightrider
    Oct 27, 2022 at 9:53
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    The converter like distrelec.ch/en/dc-dc-converter-72v-5v-fixed-recom-78he5/p/… can take from 6.5 till 72 V and output 5 V, up to 1.5W. Its efficiency is 83 %. Larger voltages are getting dangerous already.
    – nightrider
    Oct 27, 2022 at 11:43

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