>Yep, just take a rotary switch like in the photo and throw it into on outdoor junction box and done.<
That switch needs to be able to transfer the ground and the neutral seperatly.
The ground and neutral are ONLY joined at the source.
On the dock power hose, the connection is somewhere at the marina .
On the noisemaker , its at the noisemaker,
On the inverter its at the inverter.
Any rotary switch has to be able to keep the wiring to code.
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For most folks the simple 1500W inverters that are under $200 work just fine.
For the folks that can use the following & adding feature of the advanced inverters they could be worth the price.
If a 15A or 30a power source is all that is available and starting/running a second big draw that may be hard starting (2nd Air cond on 30) fine.
If a small noisemaker was preferred it too can be boosted for a while , to operate an added heavy load.
And of course if air cond will be operated from big alternators underway , a higher quality Sine Wave unit would be worth the high cost.