For the 120V side, we have 2 x 30A circuits. Originally each of the 30A circuits fed an independent electrical panel. One of the panels supplied vital electrical components like the battery charger, inverter, air conditioners, ac pumps, and receptacles for both the port and starboard sides of the boat, while the other panel supplied the hot water heater, and thats all.
When I added the electric block heaters to our heating system, I removed the second electrical panel and moved the only live circuit from panel 2 over to the first panel. With an entire 30A circuit freed up, I was able to supply both of the 1800W electrical with their own independent 15A supplies.
Essentially, Asunto now has a 30A 120V circuit that is dedicated to all the standard house loads. These include the hot water tank, AC, AC pump, microwave, and general receptacle loads (TV, computer, etc). The second 30A circuit is now dedicated to the two 15 A block heaters wired directly to two 20A circuits. This works well as we don’t have any tripping circuit breakers anymore, and we don’t need to worry about turning the heaters off to use the microwave. If we ever need any additional heat, we can always plug a space heater into the port or starboard receptacle and still have enough power for the hot water tank and microwave.
Our generator is also connected into our 120V electrical system. It originally ran both panels, but I have now rewired it to only power the primary electrical. I figured there is no need to power the block heaters, as it makes no sense to burn diesel to run a gen set to power electric heaters to heat water to heat the boat, when we can skip a number of those steps by either running the main engine through the heat exchanger or just run the diesel heater directly.
We also have an 1800W inverter tied into the main panel. It essentially powers everything but the air conditioner, AC pump and the hot water tank. This allows for AC power when we need it, but ensures we don’t drain down the batteries un-necessarily by running appliances that should not be run off the DC system.