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Comment by Xylakant | Hacker News Reader

It depends both on the lights and the dynamo. Incandescent lights used substantial chunks of your pedaling power. LED lights need maybe 5W or so. Hub dynamos are pretty efficient. For an untrained person your average power output is around 100W. That means powering the light would be around 5% of your power if you’re using an efficient hub dynamo. That’s in the ballpark of “bad chain maintenance costs more energy”

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100W is pretty high for an untrained person. Back when I was cycling regularly I could hit those numbers but not every time. I took my first ride of the season and averaged a whooping 63 watts. Stealing 5 watts from that it taking away 8% of my energy. If you account for efficiency loss in the dynamic it's probably closer to 16% of my output. That's a huge chunk

You need to take you into account that the required power output scales quadratic with the speed. Power in itself means nothing, speed is what matters in the end. If you run the math (and various cycling blogs and magazines have done so), you end up with speed reductions in the order of less than 1% or fractions of a minute per hour cycled. You’d likely gain more by wearing clothing that has less drag.

8% less power would imply between 0.92 and towards 0.92^0.5 times lower speed depending on if you are in the mainly linear or mainly quadratic region of resistance.

Older dynamos with a bulb connected were quite tiresome to propell.

The 8% are at the top of the power output - at the bottom end, the fraction doesn't matter. You'll be able to provide 5W more if you're only pedaling at 50W.

The old dynamos were a chore. But their efficiency pales in comparison to a solid hub dynamo :)

A 63 watt average must have been taking into account all the time you weren't pedalling, that's extremely low - you would struggle to ride into a slight breeze.

It's certainly possible for a leisure ride - 100W give you ballpark 20-25km/hour. The drag scales quadratic, so if you go below 20km/h, you'll end up somewhere in the region of 63W average. Peak output would still exceed 100W.

The dynamo wouldn't necessarily need to provide full power. Say your commute is 1 hour. The dynamo can provide 3W and the light draws 5W. Your morning commute would provide 3Wh to the battery. You don't use the light so all power charges the battery. During the evening, you now have at least 3Wh of battery alongside 3Wh of pedal power so you'd have 1Wh of battery left by the time you get home. A large enough battery could store hours of surplus light just based on your normal daily commute. If not battery power is available and it's night time, the light could be dimmed or you could charge the battery from USB. If the light is off, then load on your muscles from the dynamo would decrease as the battery charges to full.

hub dynamos are not particularly efficient. it's very difficult to make an electric machine that is efficient at the (very low speeds, relatively) that a bike wheel turns at. 60% sounds about right. However, safety lights use much less than 5W, and a modest but very useable headlight about 3W, so your figures are otherwise pretty close.

> Incandescent lights used substantial chunks of your pedaling power.

Not solely caused by the lights as they are about ~5W anyway (edit: the old one in my box of bike parts says 6V/3W on the metal). The wheel dynamo's are insanely inefficient and get hot everywhere which were the primary ones used with most incandecant lights.

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