Instead of a bulky and inefficent battery + inverter setup that produces 120VAV only for the laptop's and interface's powersupplies to convert that back to DC again, I'd suggest looking at external laptop batteries that offer several different DV voltage outputs and can also power the interface.
There are many different brands. Here is one that may work for you
http://www.energizerpowerpacks.com/il/products/xp18000/. It has three outputs which can be used simultaneously: 5V, 9-12V (to power your interface) and 16-20V (to power the laptop). I use the smaller capacity version as an 5VDC external battery supply for the Tascam DR2d and it works great. I'm considering using its 9-12VDC port to power my preamps as well but have not gone that far yet.
Going that would let you use a single battery supply and do so much more efficiently than an inverter. It would also make for a much more compact and simiplified rig by eliminating the external AC supplies for both interface and laptop. You'll need to check to make sure the supply will power your laptop and adapt the cables for correct polarity and plugs for laptop and interface. Many of the batteries come with a variety of adapters.