[jandrese]

Total current consumption should be about the same (or even less, thanks to lower losses at 48v), so I'm not sure why would you would need a "much beefier" alternator. It will need different windings, but overall it should be about the same size and cost.

[taylodl]

Once you've introduced a 48v system engineers will find ways to use that extra power. They've been spending decades carefully managing that power and making compromised. Now they won't have to. So yes, you're going to need a higher output alternator to keep the 48v battery charged due to the increased load.