I always keep lights and batteries around for an emergency; these happen to be my Lupine lights, some of which can run at fractional wattage for days or even weeks.
I recommend having a Lupine Piko TL Mini flashlight around and/or a multi-purpose Lupine Piko bike lamp / headlamp. Both can run for a long time at their lowest settings (30 hours!), and even at the low settings they are far brighter than ordinary flashlights.
The Lupine Piko bike light / headlamp can run for weeks at 0.4 watts / 40 lumens power on a large battery (60 hours on 2.5 Ah battery, 260 hours on 11.2 Ah battery), and that is still quite a bright illumination. Of course, one can toggle the brightness to higher output when needed.
For a keychain size flashlight with quite long runtime on AAA batteries, get the Titanium Innovations IlluminaTi. Keep a box of long-lasting lithium ion AAA batteries on hand of course.