SmartSolar MPPT 100/20 (up to 48V)

The second number on any Victron MPPT which in this case is 20 is the charging power in amps.

You can have 500w or even 1000w of solar panels in connected to the inputs, but it the MPPT charger will only ever be able to charge at a maximum of 20 Amps of current at the set charging Voltage. 

The voltage that Victron uses in there charging settings, for 12v Lifepo4 is 14.2v. You can work out the wattage, but multiplying this 14.2 x 20 which is 284Watts. 

It should be noted that many solar panels do not perform at the advertised wattage. Ebay panels can be less than half of advertised wattage. 

Most other panels usually just slightly less maybe 10-20% less in many cases.