How Long will the Battery Last

Dear Craig
Its not such a simple question. The 90W quoted is propably the 240V adaptor maximum consumption.
Depending on how you power it (invertor or a direct voltage convertor) .my guess is you will use about 30W on average. This is only a guess and I dont have any direct experience. Can you use a multimeter to measure the consumption ?
Assuming 30W is correct a 75 AHr battery has about 75 X 13= 975 What hours of power, but rember if its a normal car battery it wont have a long life in a deep cycle application even if you limit it to 50 % discharge. A car alternator will also only charge to about 80%.
Short answer (with lots of variables) about 24 hours to completly flat. But with a very short bat life.
If you limit to about 10 hours you would get a couple of years out of a starting battery.
Sorry to be vague, but there are lots of variables. Hope this helps
Cheers John

I have to agree with John. It also depends on the capacity of the battery. Bigger batts will last longer. I have an aux batt in the cruiser and it runs my fridge for about 18 hours before it trips off. There's also a seperate batt in the camper. Expensive to set up but you have to keep the important stuff cold and the kids out of your hair.
If in doubt, talk to an auto sparky.
Cheers, enjoy the hols. Keep us posted.

How long is a piece of string?

I don't know how long it would last but i can tell you that you will be able to watch 3 DVDs no problem from it, how do i know i hear you ask well we have done it.

The battery in question is a 75 A/H Deepcycle and the laptop we use is 240V only so we use a 300Watt pure sinewave inverter to power it, we get 2 hrs out of the laptop battery trouble is most DVDs are longer than 2 hrs.

Hope this helps,

Baz.