How much alternator capacity desirable for 210ah battery charging?

[PaulRainbow]

I just read quite a nice piece of sales pitch on the master volt website about their alpha pro regulators

they recommend 30 to 50% of battery capacity for the alt and I would agree that’s about right

I have about 500ah of battery capacity, so i need to fit a 150a - 250a alternator ? What's the chances of seeing 250a going into the batteries, nil, or less ? If the laws of physics were breached and 250a did somehow make their way to the batteries the alternator would need 10 hp from the engine to operate.

My setup is far from rare, lots of modest boats will have three domestic batteries and an engine start battery, so that's about 400ah. These boats will be fitted with 20hp - 30hp engines, a 30 to 50% alternator would need 5hp to 8hp, that's going to be a bit of a strain on an MD2020

I know of a boat with 900ah of deep cycle Trojans, so he needs a 300a to 450a alternator ? That's going to need 12hp to 18hp, from the 28hp engine!