^ The switching circuit is the problem, usually its semiconductor controlled or triac. Problem with commercial emergency lights is that you need to periodically check/replace the batteries, and that adds to your home maintenance routine. The old system use 6V VRLA batteries in 6V lightning, while newer LED emergency lights use 4V batteries that is not common in the grocery. As long as the emergency light is plugged, batteries tend to wear fast because most commercial charger are poorly designed, add to that VRLA wants cool environments for them to last.

If you are a DIYer, you can just buy a DPDT relay, it comes in AC powered that can be mounted in a rail. At normal condition when there is commercial power, relay is energized and the contacts going to the emergency lightning is disconneted. When commercial power turns off, relay contacts will go to normally-closed state activating the emergency light. You will be needing a used car battery or any VRLA rated atleast 40AH, 12V Led lights(common to automotive applications)-or a flourescent tube with cheap electronic ballast(similar to what autos and bus use). Charge the battery every month for about 4hours.

Going DIY has some bonus...
For prolonged blackouts, you may run a wire going to your car.
CDRKing automotive inverters can be wired too if you want to charge mobile phones.
Connect a 12V solar charging sytem..
Simple and endless possibilities.

BTW, I have this in the farm and house in the province, its running for 5+yrs batteries just wont die.