Watts measure power; amps measure current. To go from one to the other you need the voltage, because power is voltage times current — written P = V × I. Rearranged for current, that's simply amps = watts ÷ volts. It's one of the most useful conversions in any electrical job, because fuses, breakers, and cables are all rated in amps, not watts.
A worked example
Say you have a 1000 W load on a 220 V supply. The current is 1000 ÷ 220 ≈ 4.5 A. Put the same 1000 W load on a 12 V system and the current jumps to 1000 ÷ 12 ≈ 83 A. Same power, but nearly twenty times the current — which is exactly why low-voltage DC runs need much thicker cable and bigger fuses.
DC vs AC
For DC and simple resistive loads, amps = watts ÷ volts is exact. For AC loads with a power factor (motors, some electronics), you divide by the power factor too: amps = watts ÷ (volts × power factor). Most solar DC-side calculations use the simple form.
Related tools
Once you have the current, size the rest of the circuit: the cable size calculator, the fuse & breaker sizing calculator, and the voltage drop calculator. To check current ratings by cable size, see the wire ampacity cheat sheet.
Frequently asked questions
How do you convert watts to amps?
Divide watts by volts: amps = watts ÷ volts, from P = V × I. For AC with a power factor, divide by the power factor too.
How many amps is 1000 watts?
About 4.5 A at 220 V, 8.3 A at 120 V, or 83 A at 12 V. Lower voltage means more current for the same power.
Is the watts to amps formula different for DC and AC?
For DC and resistive loads, amps = watts ÷ volts. For reactive AC loads, divide by the power factor as well.