Watts to amps
Amps equal watts divided by volts. A 1,500 W appliance draws 12.5 A at 120 V but only 6.5 A at 230 V — the same power, half the current, which is the entire argument for higher distribution voltages.
How to convert watts to amps
There is no single watts-to-amps factor, which is why this needs a calculator rather than a chart. Current depends on voltage, and the same appliance rated in watts pulls wildly different currents on a 12 V battery, a 120 V outlet and a 230 V one. On AC the power factor matters too: an electric heater is purely resistive and converts cleanly, but a motor or a switch-mode supply draws more current than the wattage alone implies.
Questions
12.5 A at 120 V, 6.5 A at 230 V, or 125 A at 12 V. Voltage decides it.
No. You must know the voltage, because amps = watts ÷ volts.
For resistive loads no. For motors and some electronics, yes; a 0.8 power factor means 25% more current for the same watts.
Protective devices should not sit permanently at their thermal limit. A load running three hours or more is treated as continuous.
Yes, and more simply — DC has no power factor, so amps are just watts over volts.