This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
Wisconsin requires lamp on the front emitting a white light visible from a distance of at least 500 feet to the front. For the rear, red reflector on the rear of a type approved by the department of transportation, visible from 50 to 500 feet to the rear when directly in front of lawful upper-beam headlamps. a flashing or steady red rear lamp visible from 500 feet to the rear may be used in addition to the reflector. The rule applies whenever it is dark. See Wis. Stat. § 347.489 (Lamps and other equipment on bicycles, motor bicycles and electric personal assistive mobility devices).
The cheapest way to stay legal in Wisconsin is a USB-rechargeable front white light and a rear red light or reflector that meets the spec above. A $30 light set covers it.