Inverter Sizing and Battery Capacity

[MJMCCAND]

I need someone with more electrical knowledge than I possess to check my math.

Given:
Television 45 Watts
DVD 12 Watts
A/C Fan Only 3.4 Amps

A/C Fan Watts = 3.4 Amps X 120 VAC = 408 W

Total wattage = 465 watts x 1.2 (factor of safety) = 558 watts.

So a 600 Watt inverter would be the minimum if I want to run the TV, DVD, and fan together, right?

Now for the battery endurance. I have a 80A-Hour battery. To calculate the load on the battery I take total actual wattage and divide by 12 VDC, right? That's be 465W/12VDC = 38.75 Amps meaning that I could only run all three items for just under 2 hours. Am I doing the math right for the 12 volt system?

Thanks.