Battery question

DrewE wrote:
If this is a modern TV, it and a DVD player will use a lot less than 200W in practice, quite likely less than 100W. It does depend somewhat on the size of the television set; larger ones have bigger backlights, which is probably the biggest user of power in the TV.

The size of the inverter is of little importance so long as it is sufficient for your load.

In many of these sorts of situations, and particularly with batteries that are not new and have somewhat unknown histories, the best way to figure what sort of life you'll see is with an actual trial. Certainly some rough estimates can be made with reasonable assumptions, but empirical evidence trumps that every time.