if you’re like me and just need the noise from the fan, and not the air circulation, there are other, more efficient ways to get white noise.. some of which would have battery power available.

for instance, i use an old laptop (with an internal battery for ‘backup’) and usb powered speakers (the ones built in aren’t loud enough when the lid is closed (it runs that way). if the power goes out, the laptop runs for several hours (even with the old battery) before it hibernates due to a low battery.