Hi John, you might want to check out these threads:
http://www.iceinspace.com.au/forum/s...ad.php?t=46306
http://www.iceinspace.com.au/forum/s...ad.php?t=46574
For my 2 cents worth, you should try and avoid using an inverter to supply power for your laptop, they're very ineffecient. Mobs like Jaycar & DSE sell 12V laptop power supplies.
If you can work out your total consumption in Watts coupled with hours of use then divide by 12 will give the capacity of battery in Amp hours, probably want to add 30% onto that as safety margin: e.g.
(watts x usage(hours) = Watt hours) / 12 = Amp hours
cheers
