900w generator how big a charger can I use 20a?

[Johny London]

Regarding the Victron combi side of things, - I have the 12/300/120 and you can "set the charge current" which although only gives an example for 45a, looks like there is no reason why one could not put in a far lower setting. 

Another setting which in this case will have a similar effect is shore supply which can be set between 2-16a. The rest of course being supplied from the batteries when needed. (With Power assist set to on).

I think the bit about the generator requirement is slightly ambiguous, and might perhaps mean " to get the very most out of the charger...a minimum of xx genny is needed"

If you are using an inverter genny, you may find that if it does "trip" it just carries on running and you won't know that you are now wasting your time/fuel and possibly caning your batteries. Not sure what happens with other genny types but presume the Victron circuitry would step in if things get too out of whack - again you may need to keep your eye on things. Though, after a while you will get your settings right and know your equipment's quirks.