It also helps to understand why batteries don't have the run time they are supposed to when you do the math. Whenever you calculate run-time of a battery, if you are trying to use a small battery to do a large battery's job, the math just won't work. You'll end up running out of power much faster than you expected. So the answer is, whenever that happens, get a larger battery or a smaller light (or whatever else you are trying to run from the battery.) With a battery that's larger for the job, the math will be much more accurate.

For example, if I can run a 20W light from a 5 ah battery for 2 hours, I might think I can run a 40W light on the same battery for 1 hour. But the draw is so heavy that the battery wimps out and quits at, say, 45 minutes or something.

That's why I like the energy-saver bulbs, they put out more light with less power used.