Hi there,
We are talking about an emergency light right? One that comes on only when the power goes out due to some external failure outside the home. That means the light would be off for most of the time, like weeks/months at a time, which gives it plenty of time to charge. Then if the power goes out due to a storm in the area or something, then the light comes on. I've seen emergency lights that have lead acid built in, but that was the 6v size cells around 4 Ampere hour. I intended to go higher than that like 12v, 7Ahr min so that there would be plenty of run time if the power went down. The charge algorithm could be very gentle on the battery too, like charge once every three months (unless the power goes out of course and then it would charge some short time after the power came back on).