Gensets vs Inverters
I believe your question is what sort of duty cycle an inverter would be good for, i.e. would there be a problem running the engine for extended periods to power the inverter. Inverters generally have a continuous and a peak rating which is higher. Inasmuch as they are electronic, they won''t wear out from overuse. Most, if not all, have protection from overcurrent or overheating built in; it should not be possible to damage them by overloading them or running them for too long. It is possible, however, that the continuous duty spec might be optimistic, meaning that the unit would eventually overheat and shut itself off, even though run at or below the specification. This is a function of how large the heat sink is, the ambient temperature, and the size of the cooling fan, if any. On my Pro Watt, the cooling fan comes on after 3 or 4 minutes of powering the microwave.
I would try to talk to the manufacturer to get a sense as to how conservative the continuous duty spec is, and oversize the inverter somewhat accordingly.
When powering the inverter from the engine, I''d be much more concerned about damage from the engine due to it being run at low loads for extended periods (assuming this is done while at anchor rather than underway).
If you have an air conditioner but aren''t sure if you want to use it away from a dock I''d stay away from a generator. However, depending on the size of the air conditioner I doubt that it would be viable to run it using the engine/inverter combination. At night you''d have to worry about carbon monoxide, and you''d have to worry about causing damage to your engine due to extended running at minimal load.
If you''re planning on recharging cell phone and other batteries, be careful that the type of charger is compatible with the inverter. Most should be OK, but there are some very inexpensive chargers that can be damaged by the modified square wave put out by most inverters, for example some rechargeable shavers. As I recall, chargers which have transformers in them are generally OK, but you may want to verify. To be 100% safe, and if possible, I''d recharge cell phones, etc. directly from the 12 volts rather than using an a.c. charger. Also, a few computers/TVs/VCRs have problems with the modified square wave, they usually won''t be damaged, but may do strange things. It is possible to get a sine wave inverter (ProWatt makes), but they''re a lot more expensive.