Well, I started my tanks in the fall so I haven't had a summer to try it out, yet. But, I do have on hand two 6v lantern batteries (the big square ones with springs on top, check the camping section). Wired in series, the batteries will produce 12v that can power a common computer fan to blow on the water for evaporative cooling.
A lantern battery should have a capacity of 4 to 5 amp hours, maybe even 6 for a premium brand. Most 80mm computer fans draw between .16 and .18 amps. That means your fan "should" theoretically
run anywhere from 22 to 37 hours. Inefficiencies in the battery (no battery is 100% efficient) will reduce that by a few hours though, so you may want to have 4 or 6 batteries on hand if you expect a really long outage.
Four "D" cells can take the place of each lantern battery, but it would be a pain to string 8 of them together to make 12v.
Or heck, just connect the fan directly to a car battery. It would run for weeks!