Be aware that power supplies are almost always advertised for peak amps, not continuous amps. True of everything from wall warts to megabuck industrial supplies. Linear or switching design.

Sure, your supply may appear to be fully capable of producing the full amps, maybe even a touch more. It should, for a while anyway. But at the cost of more voltage ripple, voltage sag, heat, and reduced life & reliability.

I agree with Duster, the charger inefficiency can certainly be greater than 20%. It will typically vary a fair amount depending on how much of its maximum power is being used for example.

In my opinion this is not a place to scrimp, I suggest getting something larger than it appears is needed. Not to mention most of us end up adding additional chargers over time anyway.