If you are going to rely on inverters you better have a big battery bank and a way to charge it. The alternator size is important, but also the sophistication of the regulator is too. With twin diesels you could put on 150 amp alternators, with sophisticated regulators, and a Centerfielder so they both put out. You can buy inverter chargers with 150 amp charging capacity. And don't forget solar, 99% of the Manta's have big solar ie. 500 watts, on a bright day we will see 20+ amps out of those. All of this combined will usually take care of the 12 v needs on a sail cat. With a power cat when you are usually running more, the alternators are a big factor. Putting a 150 amp charger on the genset, with the water heater, or one ac is usually plenty of load. Great time to do the laundry too.