Comment by Aardwolf

Not having to take the light off the bike and charge it and then forget to take it back to the bike, not to mention forgetting charging it and finding out when it's dark, is completely worth having a dynamo.

It's not about weight, it's about having the light work when you need it.

Ensuring the battery is not empty at the time you want to ride it and it is night is not always convenient

I'm talking about commute, not sports, here.

They make a ton of sense when you’re riding long distance and when you don’t have access to a charger.

Exactly my setup, a nexus hub with 8 speeds, the bike has been outside under rain and occasional snow, and it's completely reliable for my daily work commute.

> Which bike to put it on?

As mentioned, a city bike, I don't think a dynamo hub would be a practical choice for a racing bike or mountain bike, nor fit all riders.

The drag is not an issue for a bike that I use to commute to work and go shopping, the convenience out weights by far the drag, and if the worry was weight and drag there are other parts that contribute much more like the carry bags, pannier rack, mudguards that again, are convenient.

Another positive factor not mentioned would be environmental, no need to create waste with batteries.

I would imagine it's minimal. I'll never forget a demo I played with at a Science museum as a kid. A dynamo with a crank you can turn, and set of switches that allow you to turn on one, two, or three incandescent bulbs. The crank turns freely without load, and is increasingly more difficult to turn as load is added.

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

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.