Your best bet would to take a look at the output of your power adapter and base a battery system on that.
Also, to squeeze more battery life out of the machine look at lowering the screen brightness and cpu speed when recording. Unless you're doing realtime effects, you don't need a quarter of the processing power.
Even better - record with the display shut (set Windows to do nothing when the display is closed). This will conserve mucho battery power because its not running the screen at all.