Watts = Amps x Volts

Your power supply is 75a x 12v so it's 900 watts, but you won't necessarily be using all 900.

14.8v x 6a = 88.8w
7.4v x 8a = 59.2w
7.2v x 5a = 36w

Conservatively, you'll likely use less than 250w for your charger. A box fan shouldn't pull more than 100w or so. A 1kw (1,000 watt) generator should more than do it.