300W power supply with 110VAC input, and 12VDC output, what would be the max safe amperage you could draw?

I understand the "electrical triangle" I.E.... watts\volts=amps etc. but in this situation would you use the voltage from the ac or dc to calculate the amperage? I.E... 12x=300, Or maybe 110x=300.? (where x = amperage)
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