inverters

It will be ok.

Arfa

So long as the extension cord is capable of handling the draw of the grinder then this will be no problem. The inverter capacity isn't part of the equation.

stuart hath wroth:

That depends on whether your 600 watt inverter can deliver 500 watts at 100% continuous duty cycle. Most of the cheap mobile 12VDC inverters cannot and will get toasty hot. If it's a tiny little portable inverter, it's gonna blow. If it's a big ugly heavy monsterious huge monster, with oversized heat sinks, and a noisy fan, it will work just fine.

There's a small chance that the extension cord may be a problem. If it's of insufficient wire guage, the voltage drop will be excessive and your grinder will run slow or grind without much torque. Just about anything bigger than #16 awg will work, but there's always someone that wants to use a roll of zip cord as a long extension cord. What's the wire guage?