First of all, you need to confirm the size of the bike wheel to choose the corresponding kit (it should be noted that the 28-inch kit can be used on 28-inch, 29-inch and 700C).
Followed by choosing the right power for you, as a city commuter then choose 250w kit, if it comes to mountain cross-country then you can choose a higher power.
Finally, confirm the mileage of a single ride as a way to choose the right battery.