My goal for this article is to help you choose products that will meet your needs and be at least moderately reliable. Many, but not all, products have been tested in heavy rain. Some advantages and disadvantages apply to entire categories. For example, all generators slow you down, but unlike batteries they rarely die when you're half way home. Unfortunately, current LED lights aren't bright enough to illuminate the road, so you need at least one incandescent bulb facing forward.
Generators get energy when pressed against the spinning tire.
Generators can be disengaged when you don't need any light. A generator adds drag and slows you down, although the good ones don't slow you much. Generators don't get a good grip on some tires, especially when the tires are wet. Generators require more skill and patience to install than do most battery-powered lights. Even when new, none of the generator lights I've owned have been as bright as I would like.