If you’re talking about, say, a 1/2” impact wrench, those are in the 7-8’ amp range. You would need something closer to 800-1000 watts. You’re probably better off getting a good 20v cordless impact, they’re cheaper than a 1000w inverter.
Edit: Then you can charge the cordless battery with the 400 watt inverter.
2018 Redwood 3901WB Autumn Fire
3 A/C, Heat Pump, Dishwasher, Washer/Dryer, Disc Brakes, Onan 5.5kw
2017 Ram Longhorn 3500 Cummings/Aisin DRW LB CC and a cute little aftermarket fuel cap