I have a couple of those big candles in glass jars about 10" tall and the diameter of a coke can. They burn for about a week nonstop. They are about $1.39 if you shop carefully and $2.00 or so if you don't. Either way, it's hard to get more cost effective than that, if you don't mind candles.

Aside from that, you want a few LED flashlights. There's lots of discussion of those already so I won't get into it here.