Here is the table of recommended maximum power ratings:
The charge controller will only pass its rated amps which is the second part of the model number. 100/20 means 100 volts maximum input, 20 Amps maximum output. If you have more power available the excess is wasted.
If your battery was flat (12 volts) and you had 300 Watts of power available then the formula Watts=Volts x Amps gives you 300 Watts = 12 Volts x 25 Amps In this case 5 Amps (20% of output) is being wasted.
This is an extreme example, normally the battery voltage is higher, see below
When the battery gets up to 13 volts then the numbers become 300 Watts = 13 Volts x 23 Amps so you are still wasting power but not as much
At 14.4 volts the numbers become 300 Watts = 14.4 Volts x 20.8 Amps so we are still wasting 0.8 Amps
Solar power is expensive so we dont want to waste any, it doesnt pay to try and economize on charge controllers.