Interesting question, so I set up a little test. I have a new T7 system, whose motor is rated at 1.65A @ 115VAC. To make sure the motor didn't exceed that (i.e., to make sure the ratings on the motor were reliable for use in calculating for an inverter), I ran the machine under different conditions while monitoring the current with a Fluke 289 DMM and a calibrated Fluke i410 current clamp.
Running with a wet wheel and no additional load, the motor consumed 0.9A. I then simulated a usage load by bearing on the wheel with the coarse side of the stone grader, with a lot of pressure, such as I'd use when re-grading the stone. The maximum current observed was 1.35A and I could only make it go that high for less than a second. My household voltage runs around 120V, so I would say the 1.65A @ 115VAC rating on the data plate is spot on accurate, and maybe a tad conservative. (Increased voltage = decreased current, all else being equal.)
So, let's use the rated 1.65A for inverter calculations. Inverters are rated in watts, not amps. 1.65A @ 120V is 198W. SO - in theory, a Wal-Mart 300W inverter running from your cigarette lighter plug would give you plenty of capacity with a 33% margin (and by the way, margins are GOOD things where inverters are concerned - never use one that barely meets the load requirements).
But there's more to it than just watts. There's also the quality of the power to be concerned about. Small AC motors are more forgiving about power quality than a lot of appliances (cheap inverters can actually destroy some types of electronic devices), but it's still something to be aware of. AC equipment is designed to run on "true sine wave" power. Virtually all cheap inverters don't produce true sine waves; they produce an artificially approximated sine wave. Depending on the manufacturer, they'll call it a stepped sine wave, a PWM sine wave (hoping you won't look up the definition of "PWM"), a square wave, or a "stepped approximation to a sine wave." They all mean the same thing. For your purposes, the important point here is that the cheaper the inverter, the less it's power appears as a true sine wave to your motor. Again, electric motors are more tolerant of this, but if this is going to be used frequently (commercially?), then you might do your machine a favor by spending more for a better quality inverter. You don't need that much capacity - again, 198W is a small inverter - but by getting one with some extra capacity with a better sine wave, you might find that you can run another small appliance or two with it and be better off. A better quality unit will also run cooler, more efficiently (i.e., run your machine longer between needing to re-charge the battery), and generally last longer.
There are several decent brands of inverters. Xantrex and Tripp Lite come to mind, but there are so many companies making them now it's hard to keep up with them. Another option, albeit an expensive one, would be a small inverter generator such as the Honda eu2000. At 200W you could run all day long on a small tank of gas.
