The issue I'm facing is calculating current draw from different household items. Like, if a fan is rated at 75 Watts, its current draw would be A = W/V so A = 75/220 and A = .34 amperes.
That doesn't sound right to me, because I have two 150 A-h batteries connected in series to an inverter/charger. Assuming that I use up only 60% of the batteries' capacity to prolong their life, I have 180 A-h of battery capacity. And also if the inverter is only 80% efficient, I have 144 A-h of usable battery capacity. If the fan does indeed draw .34 amperes of current, that would mean that this battery setup should run the fan for around 424 hours.
I know I'm doing something wrong, I just don't know what it is. Could you guys help me out here? What's the backup time for a 75 watt load drawing power from an inverter connected to two 150 A-h batteries in series?