How many amps (dc) will a 400W inverter pull in my car?

I am just curious as I do not know how inverters work. If I have a 12VDC-110VAC inverter in my car running at 400W continously what will my DC amperage draw be that powers the inverter. Do I divide 400W by 12V to get a continuous amperage draw of 33.3 amps @ 12VDC. Is this right?
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