Inverter output is rated in watts. Some appliances are rated in watts and others in amps. To convert from amps to watts multiply by 120 (voltage). Add up the watts for all the devices you'd want to run at the same time and get an inverter rated to handle that load. I suggest choosing a model that's 30-50% higher than you need for a safety margin.
Now things with a motor in them are a potential problem. Is the amp rating the maximum start-up draw or nominal running draw? I don't know, I've never tried to run something with a motor off an inverter. The rating air conditioners I know is the nominal running power, start-up draw to get the fan and compressor motor in an AC is 2.5X to 3X the normal power draw. My guess is that for tools the max amp draw is what is stated on the tool. Also, if the device has a built-in fuse or breaker that would be a reliable indication of the maximum draw.
Be careful to mount the inverter someplace protected from water. I'd be concerned about mounting one under the seat. Off-roading through deep water or just a leaky top can result in a lot of water collecting under seats, which tend to be the low point in the tub. Also make sure to use the proper gauge wiring for the load and distance to the battery, and put a fuse or breaker in-line in the wire as close to the battery as possible.
When choosing an inverter try to get one that puts out a sine-wave (best) or modified sine-wave (not as good) waveform, has low-voltage cut-off (to save your battery from being run down too low) and GFCI protection (to prevent electrocution). These tend to be more expensive, but it's worthwhile to protect your life and your rig.
I'm a JKO Reject
Last edited by BumpInTheRoad; 08-11-2012 at 09:02 PM.