https://www.bike-components.de/blog/en/2021/11/your-custom-wheel-with-a-hub-dynamo/
Choosing the right hub dynamo
When it comes to hub dynamos, there are more aspects to consider than with conventional hubs. Power output is an important feature, and all models produce a voltage of six volts – however, they differ in what power they deliver. Until a few years ago, the StVZO stipulated a nominal voltage of six volts and a nominal power of three watts, and practically all dynamos available in Germany met these criteria. The value was reasonable to operate halogen lighting. Since the strict regulation from the StVZO has been dropped and efficient LED lamps have replaced other light sources, there are also hub dynamo models with a lower output of 1.5 or 2.4 watts, which are lighter and rotate with less resistance. Whether this is a sensible option for you also depends on the light you choose. There are models that reach their maximum light output with 1.5 watts and those that can only be fully exploited with a three-watt dynamo. For everyday use in the city, 1.5-watt combinations are adequate. For cross-country trips or passages in the forest where maximum light output is important – even at low speeds – we recommend three watts. Since the rotational frequency at the same speed depends on the wheel size, there are hub dynamos that are optimised for this purpose – for folding bikes, for example. Most of them are constructed for universal use, but it is always worth taking a careful look at the description.
