Check with a solar specialist that is not associated with your power company. I'm guessing that you are set up to sell the excess capacity back to the power company. If so, it's set up so that when the grid is down, you don't feed power to lines that they think are cold. A solar specialist or electrician should be able to set up something like the transfer switch used for a generator.

IIRC off grid solar uses a battery bank & charger to smooth the voltage being fed to an inverter.