The big loads wired to the new isolated section of the breaker panel will want to be be powered by either the inverter or the grid,depending on the availability of the sun to keep the batteries up during the high current draw required.
Need to remember that to get 10 amps at 240VAC (2400Watts) from the inverter,the batteries need to put out 50A @ 48V.
To carry the high AC currents I needed a big DPDT relay or other safe way to switch the load L1 and L2 lines to the desired power source.

After not having much luck looking on line for a big generator transfer relay, I dug through the parts salvaged from the big motor driver units I got several years ago, and found just what I was looking for.
This dual relay is designed to reverse a big 3 phase motor, so it has all the contacts required to do my grid to inverter transfer.
List price is $600, so I just saved a lot of money since I got it for free, and got a very high quality industrial relay well suited for the task. It features both mechanical and electrical safety lockouts to prevent both relays from being activated at the same time.
A good example of why saving cool components from the scrap yard can save you a pile of money.