Just for something to do, I took my Stanley tripod light, and put in nine rather old Rayovac AA Nimh cells. I think the light is half watt LED. Enough to be able to walk around a room comfortably. I set it up, and let it go. After about two days, the light went dim. I pulled all nine batteries. Five were still some what charged. Put three best ones back in one of the tubes. Ended up getting three days of light out of the old NiMH cells. Of course in most situations, I'd only need a couple hours here or there. But, it was a fun test.
Now to recharge all those cells before the real power cut.