When it comes to selecting which beam angle you need there are certain things you need to consider:
How far apart are your lights/ How many lights is there?
If your lights are very close together or there is a lot of them, the beam angle should be quite low so aim for around 20°. If your lights are spaced really far apart and need to light the whole room 36° and over are probably the best choice.
How high is the ceiling?
The more distance between the floor and the ceiling means the wider the beam angle and the brighter the light. To see how bright a light is you have to look at the lumens. The amount of lumens indicates the brightness of the light, the more lumens the brighter the light. However, it is important to remember that though lumens remain the same, a larger beam angle will provide more coverage but a diluted light whereas a smaller beam angle will produce a brighter more concentrated light. When lighting a shelf a narrow beam would be required such as 20° but if you were lighting a room (from ceiling to floor) a wider beam angle would be required as would more lumens.
Where do you want the light to focus?
This again comes into ceiling height to a certain extent. A general guide is: For work surfaces, such as the kitchen bench, aim for a beam angle of around 20° with a lot of lumens for a clear bright light. For lighting a whole room from the ceiling you would need a beam angle of over 30° to spread light throughout the room. When lighting a display cabinet, 15° or under should be enough to spotlight your chosen items.
How bright do you need the light?
This can depend on the room, the ceiling height, the purpose of the light and personal preference. For example, a kitchen would probably need quite bright light in order to see clearly when chopping vegetables/preparing a meal but for a bedroom or living room you might want a more diluted light as those rooms are for relaxing,