400 to 500 watt is ample, I have one that is rated at 1000w but given it's size I'd be very surprised if it could do any more than 500w continuous, in saying that though it's worked fine for charging/powering a 17" laptop, charging phones, and other things.
One thing to watch out for is the advertised wattage, seeing more and more inverters advertising their peak wattage and not their continuous wattage, so you might be buying a 500 watt inverter until you read the fine print and see its only good for 300 watts continuous.