Watts to Amps Calculator
Convert watts to amps for AC or DC circuits with optional power factor.
Watts to Amps Calculator
Converting watts to amps answers how much current a load draws at a given voltage. Watts measure power; amps measure current flow. The link between them is voltage—and on AC circuits, power factor.
This calculator works for DC battery circuits (12, 24, 48 V) and AC mains (120 V, 230 V). Enter watts and voltage, set power factor to 1 for pure DC or resistive AC loads, or lower it for motors and other inductive equipment.
Use the result to size fuses, breakers, wire gauge, and inverter input ratings. Pair with the amps-to-watts calculator for reverse conversions, and with wire size or voltage drop tools once you know the current.
How this calculator works
- Enter power in watts—the real power the load consumes or the inverter supplies.
- Select system voltage: 12/24/48 V for DC, or 120/230 V for AC as appropriate.
- Set power factor: use 1.0 for DC and resistive AC (heaters, incandescent). Use 0.7–0.95 for typical motors and appliances if you know the PF.
- DC formula: amps = watts ÷ volts.
- AC formula: amps = watts ÷ (volts × power factor). Result is line current in amperes.
Core conversion relationships
DC: I = P ÷ V | AC: I = P ÷ (V × PF) | P = watts, V = volts, PF = power factor
Power equals voltage times current in DC and in AC when power factor is included. Dividing watts by volts gives DC amps directly. On AC, apparent power is volts × amps, but real power is volts × amps × power factor—so amps = watts ÷ (V × PF). A power factor of 1 means all apparent power is real power (unity PF).
Formulas used by this tool
- DC: amps = watts ÷ volts.
- AC: amps = watts ÷ (volts × power factor).
- Power factor defaults to 1 when left at unity.
Worked example: 1,500 W at 120 V
A space heater draws 1,500 W on a 120 V AC circuit with power factor 1.0 (resistive load).
- Amps = 1,500 W ÷ (120 V × 1.0) = 12.5 A.
- A 15 A or 20 A branch circuit is typical; verify against local code and continuous-load rules.
- On 12 V DC, the same 1,500 W would draw 1,500 ÷ 12 = 125 A—why high-power DC needs thick cable and large inverters.
- If the load were a motor at PF 0.8: amps = 1,500 ÷ (120 × 0.8) ≈ 15.6 A—higher current for the same real watts.
Practical tips
- Use nameplate or measured watts, not guesswork—especially for variable-speed and cycling loads.
- For DC solar wiring, convert array or inverter watts to amps before using wire size calculators.
- Continuous loads (heaters, pumps) may need 125% ampacity margin under some electrical codes.
- Power factor 1 is correct for DC and most resistive AC; when unsure on motors, use 0.8 as a planning estimate.
- Higher voltage means lower amps for the same watts—one reason off-grid systems move to 24 V or 48 V.
Frequently asked questions
What is the watts to amps formula?
For DC: amps = watts ÷ volts. For AC with power factor: amps = watts ÷ (volts × power factor). Example: 600 W at 12 V DC = 50 A. 1,200 W at 120 V AC at PF 1 = 10 A.
When do I use power factor?
Use power factor on AC circuits where load is not purely resistive. Motors, compressors, and fluorescent ballasts draw more apparent current than real power suggests. DC circuits and resistive heaters use PF = 1. Leave at 1 unless you know the load’s power factor.
Does this work for three-phase?
This tool uses single-phase math (one voltage and one current). Three-phase uses different formulas involving √3 and line-to-line voltage. For three-phase planning, use phase-appropriate calculators or consult an electrician.
Why do amps matter for wire sizing?
Wire and fuses are rated in amps. Exceeding ampacity overheats cable. Once you know watts and voltage, converting to amps lets you use wire size and voltage drop calculators and pick breakers that match load current.
Watts vs amps—which should I size from?
Appliances are often labeled in watts (power). Conductors and overcurrent devices are rated in amps (current). Convert watts to amps at your system voltage to bridge equipment specs and electrical installation limits.
Can I use this for inverter sizing?
Yes, as a first pass: sum load watts, convert to amps at battery or DC bus voltage for DC-side current, or at AC output voltage for AC current. Add headroom for surge loads (motors) beyond steady-state amps from watts alone.
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