cheapest/simplest way (without going grid-tie) is to have an electrician add a critical loads panel supplied
Cheaper way is have electrician wire a manual transfer switch at the existing panel. When you loose power, turn off non-essential breakers and then flip transfer switch.
You lose all benefits of solar/battery except for during a power outage and you have to flip all your breakers? (you also won't immediately know when the power is back).
Might as well save money and not install anything- use an extension cord for those rare times.
Our solar inverter uses the 60hz AC from and grid to do the DC->AC conversion. The inverter stops functioning if the power is out. I thought they all did that for safety.
Cheaper way is have electrician wire a manual transfer switch at the existing panel. When you loose power, turn off non-essential breakers and then flip transfer switch.