Oddly enough, I own another company that services the fuel for emergency generators, so this thread got my attention.
I can tell you that if the grid went down for an extended period of time, you'd have more to be concerned about than whether or not your sign business survived. While much of big business and government have generator systems that are supposed to kick in in the event of failure, most overlook the need to maintain the fuel that feeds those generators. Further, under such circumstances as a prolonged grid failure, everyone overlooks the fact that the ability to refuel will also be effected. Much of the problem is caused by the reformulation of fuels in the past few years that make the fuels far less tolerant to long term storage.
For those of you with the belief insurance will cover our losses, you need to take a better look at your policy. Most insurance policies have clauses that limit claims caused by Acts of God, Terrorism, or other lame excuse to not pay out.
Like Pat said, the need for signage would be minimal under such circumstances, and since ATM's, POS terminals, and such would also be down, how would you get paid for the work anyways? Who carries cash anymore?