Let's start from the goal of having good lights: the goal is to see ourselves but also to be seen. This is why we must also pay particular attention to the rear light, especially if we find ourselves using asphalt roads (as well as being required by law).
TYPES OF LIGHT
There are three types of bike light: classic, halogen and LED. There classic It's just for the record but it's no longer used in sports (unless you want to create a vintage effect). There halogen bulb it is a nice compromise, which allows greater control over the light beam and a power between 0,28 and 0,44 Watts, but the most used (and recommended) is the LED light. THE LED bike lights made with semiconductor materials, which emit photons when an electrical voltage passes through them. The advantage of the LED is greater power with lower consumption, furthermore the light beam is easily controlled thanks to a system of mirrors inside the bulb which directs and amplifies the light.
In the lights on the market we find, as a reference value, lumens or lux. The lumen is the unit of measurement for the power of the luminous flux; lux instead expresses the width of the light beam (the quantity of lumens per square meter). Two important values given that on a bike we need power but also width (example: to see the edges of the path).
If you use the bike in the city in fairly well-lit areas, fixed or intermittent lights between 20 and 30 lumens are fine, but the power must reach 200 lumens if the streets are less lit. For the rear lights, 30 lumens are sufficient. Don't forget the width of the light beam because in the city peripheral lighting is essential for your safety.
If you practice road cycling you have to be seen, so the power of the rear must be at a good level (it cannot be less than 100 lumens). At least 500 lumens at the front.
If you are an off-road lover instead, is the most demanding type of biker, because it must illuminate both in depth and peripherally. My advice is at least 1000 lumens and a light beam wide enough to illuminate the sides. The ideal would be to combine a larger handlebar light with a more directional light on the helmet (at least 500 lumen), so as to have achieved both objectives.
WHERE ARE THE LIGHTS FIXED?
Rear lights usually attach to the seat post or it can happen directly to the saddle shell (like some models). Be careful not to put it in a position that will then be covered (think of the vertical tube of a full suspension where, in case of shock absorption, it is covered by the rear wheel).
When off-road, it is advisable to have a light on the handlebars, to illuminate short-distance obstacles well, and one on the helmet, to illuminate longer ranges and be able to direct the light.
The models we recommend have a rechargeable battery via USB (then we find batteries or dynamo). There are also specific lights for e-bikes that use the bicycle battery as power (safer system to avoid being left without light).
Rain exists in the world, especially in winter, and therefore it is very important to check that the waterproofness of the lighthouse is good. The reference standard for water resistance is called IPX, the maximum is IPX7 (at least IPX6 is recommended).
As a final piece of advice, especially for longer outings, a power bank. If you run out of battery, a short coffee break to allow you to get home safely. As long as the bars aren't closed!