I came here to suggest something like this, a big ‘power bank’. That CHAFON model states it has 31 AH (amp-hours) and also has a port for solar charging - bonus! So, if you pull less than 10 amps, then you have about 3 hours of battery. Looking around online a 21-inch box fan on high pulls maybe 16 amps, and 10 amps on low. A trainer wont pull hardly anything, - right? So that CHAFON would get you about 1.5 hours with a fan on high. You could also look for a used UPS if you need a deal. Just haul it out to the shed, use it, then haul it back to the house, and recharge it for next time.