Sorry to be verbose, but the shore power comments make me recall another important idea as all of the electrical works in concert.
In my neck of the woods it's heaters that draw the high loads, not summer ac , but ultimately the same problem. Every year we burn boats down when shore power connections catch fire. This is very common, number one cause. Many more with close calls with melted plugs. I noted long ago when I trade up boat sizes that the 30 amp marinco cords that used to work, suddenly we're problematic. You can run one heater on a 30 amp cord successfully, or even two if they are both intermittently cycling, but once the boat is large enough that two heaters are running at long duty cycles then the shore power connections become problematic. The difficult problem being a majority of 50 foot slips if not brand new have 110/30 power, not 220/50. So you unavoidably have to suck that power through a small straw. Unless those 30 amp connections are perfect,they just don't stand up to anything close to a more continuous 30 amps of delivery without overheating. They just don't.
Many of us went to the smart plugs as they are a much heavier design to try to combat the melting connections. What we found this past winter after a few years of comparing notes among many is that the smart plugs were melting too! The common theme with the smart plugs was it seemed to be with the versions where you put the smart plug on an existing cord. Smart plug beat up a friend a bit more than warranted claiming they were getting installed wrong. Could be, but now that I know empirically that multiple anal retentive types are apparently not able to put these together right, at least one an engineer, another an electrition, maybe it's not possible for the consumer to put these together. Sorry smart plug, I really do love you guys and have stood in your office on multiple occasions as you supported my boat club, but you just walked into this one when you try to blame the consumer. Can't protect you from yourself. You should have investigated who you were talking to before blaming them. In any case, of the pre-molded complete cord and end, we have not seen this problem repeat. So most of us have bought the pre molded cord set now, seems to be working and prices have dropped too.
Sorry, I told you this was going to be verbose.
In summary, when pulling 30 amps more or less continuously on a 30 amp system, even when using upgraded components, it seems we are still on some kind of razors edge. Upgrading the shore power would be the real solution, but that's often not available. Plus, if you cruise north around here, it's not uncommon to run into 20 amp shore power, or a standard household 15 amp which you can't get anywhere close to 15 after the dock dogs pee on them. All of this very common, real life.
So, that's where I got my inspiration that my 48 foot boat somehow needed to be able to get by on much less than 30 amps of power on a typical day. If I can't fix the shore power capacity, and I can't fix the 30 amp connections to be bullet proof, the only thing left is to reduce the load on the cable itself.
That gentleman is why I went with a phase matching inverter. The simple feat is that I can manually select the max current of the shore power inlet. That's a configurable setting. So I set mine on something less than 30 amps, usually 20. In an instant, I never had a shore power connection overheat again. In a raggedy marina with household outlets? I just select 10 amps. I've done it, it works not just in theory, but real life.
The magic why this works is that you only need your shore power to keep up,with your average load, which is much less than your peak loads in most cases. If so, then this system and it is a system, not just one component, it will work wonders. On startup loads, the inverter adds a little more than what you get from the shore power alone. In a while when the peak load drops, the charger goes to work putting back what you just took out. I've arrived at a cold boat and turned on two heaters and the hot water heater at the same time and in 4 hours time the charger had already replaced the enormous load I had taken out when I got there.
The down side? Cost. Batteries. Wiring. These units are not cheap, you need to have a sizable battery bank and often our old wiring finally presents itself as the number one project on the list. All things I would have done eventually anyway, bug they all had to be present together for it to work.
Now that I worry less about my own boat burning down, I just have to worth about all the boats around me.
