your batteries and how fast. It's a matter of how much it costs you to do it and the amount of time it takes. There are several ways to make amps and combinations of ways to make it even more cost effective over "time". If you're long term cruising then running the main engine for any length of time to charge batteries is costing you too much. I can burn the engine out of my generator and just replace the engine several time for the cost of just one main engine overhaul. Not to mention that it burns 1/3 of the of fuel. Not to mention the side loading of the shaft due to the extra pull of the larger alternator. You could have a pretty long list of all the reasons not to use the main engine. Having said that, I have twin alternators on my engine. I can put out about about 150 real usable amps while at cruising speed but unless I want to use the inverter to run the toaster, coffee pot, hair dryer and vacuum cleaner then you need some kind of generator anyway. 3000 watts is enough to run everything I need. I can't charge the batteries and run the air conditioner at the same time but if I manage the loads it's big enough. My goal is to be able to sit in the anchorage for a week or two and never have to turn on the main engine. When I'm motoring from place to place, well, I just consider those "free" amps.