Late last year I received an extra gift from my mother. It was a package with four battery backup light bulbs. These bulbs have a battery built in, so when the power goes out I will still have light.
The bulbs I received are from a company called BrightLiving. The instructions indicated that you need to leave each bulb on for 8 hours to charge the battery, and that once charged the light would work without power for 3 to 4 hours. (Interestingly enough, the video on their web site says their battery bulbs can last up to 6 to 8 hours.)
The extra benefit is that, even though power is off, you can still control the light from the wall switch. (The package also came with two control items so you can use the charged bulbs as a separate light. Why you would want to is beyond me, as a flashlight or lantern would probably work just as well.)
I have put one bulb in the living room, two in the kitchen and one in the bathroom. With careful use (as in relying on other light sources for general usage) you might get several days use before the battery runs out of juice. (How long do you need to be in the bathroom at night?)
I went looking on Amazon for battery backup light bulbs and found several brands available. They start in price from $5.49 for a single bulb to $64.95 for a package of four. Most of the ads say they can last 3 to 4 hours without power.
Looking at what was being offered I noticed that some of the bulbs indicated that they can be controlled from the light switch when power is out, and others did not mention this. I think this would be important, as with our recent blackouts, you would want to be able to extend the use of the bulb over several days.
I have not (yet) had the opportunity to see how well these bulbs work in a blackout situation. Only time will tell.