I have two 12V fans in my boat. One West Marine one that tends to blow a descent amount of air and then one that is mounted in my V berth that really don't move all that much air. That being said, the one on the V berth isn't that expensive and it does move enough air to help with hot temperatures. I also like the white noise of a fan running so I tend to run mine a lot.
I have a two battery house bank that are group 27s. I do not run refrigeration on my boat so there isn't much power consumption going on. I have my house bank on a Link 10. I can run the fan in the V berth all night, the cabin fan for most of the afternoon, a small inverter to power the lap top, and the anchor light and I am usually at 98% or 97% in the morning when I get up, according to the Link 10.