You could run it off of two nine volts if you wanted to, the only problem is thats 18 volts, which your fan may or may not run on. You would have to wire a 12 ohm, 3 watt resistor in to make it 12 volts but that would waste energy and cost money. Also the batteries will not last very long and it would get very expensive. If you wanted to run it on disposable batteries I would get 2 of those huge 6 volts as they will last much longer. By far the best way to go would be to get a 12v lawn mower/utility battery for 20 bucks at wal-mart. The only problem with that is you would have to charge it fairly often. If you bought this, http://www.siliconsolar.com/shop/cat...ger-p-133.html or something like it you would keep the battery charged by solar power. Since the fan is a motor and motors drain batteries like no other I would highly suggest not using a battery. Well anyways hope that helps