Watts to Amps Calculator – and kW to Amps
Convert watts to amps or amps to watts for DC, single-phase AC and three-phase AC circuits. Enter power, voltage and power factor to get current, apparent power and suggested breaker size.
Convert Watts, kW or kVA to Amps — or Amps Back to Watts
Watts to Amps Formula
Where:
- \(I\) — current in amperes (A)
- \(P\) — active power in watts (W)
- \(V\) — voltage in volts (V): line-to-line for the three-phase line-to-line case, line-to-neutral otherwise
- \(PF\) — power factor (1.0 for DC and purely resistive loads)
- \(k\) — topology factor: \(k = 1\) for DC and single-phase AC, \(k = \sqrt{3} \approx 1.732\) for three-phase with line-to-line voltage, \(k = 3\) for three-phase with line-to-neutral voltage
The two formulas express the same relationship from opposite directions: current is power divided by voltage (and, for AC, by power factor and the topology factor), while power is current multiplied by the same quantities. The relationship \(P = V \times I\) is a direct consequence of Ohm's Law.
Watts to Amps – Calculation Example
Example: a 1200 W resistive heater runs on a single-phase 120 V AC circuit with a power factor of 1.0 (the calculator's default values).
\[ I = \frac{P}{PF \times V} = \frac{1200}{1.0 \times 120} = 10 \ \text{A} \]
Result: the heater draws 10 A. The apparent power is \(S = V \times I = 120 \times 10 = 1200\ \text{VA} = 1.2\ \text{kVA}\). Because a continuous load may use only 80% of a breaker's rating, a 10 A load needs a breaker of at least \(10 / 0.8 = 12.5\ \text{A}\), so the next standard size is 15 A.
How to use the calculator
- In the Convert dropdown choose the direction: Watts → Amps or Amps → Watts. Only the relevant input field is shown.
- Select the Circuit type: DC, AC single phase, or three-phase AC using either the line-to-line or line-to-neutral voltage. This controls the topology factor \(k\) and whether the power factor is used.
- Enter the known quantity: Power (P) in W or kW, or Current (I) in A or mA.
- Set the Voltage (V) from the list of common system voltages, or pick Custom… and type your own value in the Custom voltage (V) field. For three-phase line-to-line, enter the line-to-line voltage (e.g. 400 V, not 230 V).
- For AC circuits, enter the Power factor (PF) between 0 and 1. Use 1.0 for heaters and incandescent lamps, typically 0.8–0.95 for motors and other inductive loads.
Read the result, along with apparent power in kVA, a table showing the same load at other common voltages, and a suggested breaker size based on the 125% rule for continuous loads.
What the conversion means
Watts measure power — the rate at which electrical energy is consumed — while amperes measure current, the flow of charge through the conductors. For a DC or single-phase resistive circuit the link is simply \(P = V \times I\), which is the rearrangement of \(I = P / V\): at a fixed voltage, more watts always mean more amps. The conversion matters in practice because wiring, breakers, plugs and fuses are rated in amps, while appliances and nameplates are usually labeled in watts.
On AC circuits two extra factors appear. First, the power factor: inductive loads such as motors, transformers and fluorescent ballasts draw current that is partly out of phase with the voltage, so only the fraction \(PF\) of the voltage–current product becomes useful power. The remaining portion is the reactive power, and the total product \(S = V \times I\) is the apparent power in volt-amperes. That is why a 1200 W motor with \(PF = 0.8\) draws more current than a 1200 W heater at the same voltage.
Three-phase systems distribute power over three conductors, adding the factor \(\sqrt{3}\) (line-to-line) or 3 (line-to-neutral) to the formulas. Be consistent with which voltage you enter: 400 V line-to-line corresponds to 230 V line-to-neutral, and mixing the two changes the result by exactly \(\sqrt{3}\).
The suggested breaker size follows NEC 210.20(A): a continuous load (on for 3 hours or more) must not exceed 80% of the breaker rating, so the breaker must be at least 125% of the load current. For a full sizing workflow including conductor ampacity, see the Circuit Breaker Size calculator.
Common watts to amps conversions
Reference values for single-phase AC with \(PF = 1\) (a resistive load):
| Power | Current at 120 V | Current at 230 V | Current at 240 V |
|---|---|---|---|
| 300 W | 2.50 A | 1.30 A | 1.25 A |
| 600 W | 5.00 A | 2.61 A | 2.50 A |
| 1200 W | 10.00 A | 5.22 A | 5.00 A |
| 1800 W | 15.00 A | 7.83 A | 7.50 A |
| 2400 W | 20.00 A | 10.43 A | 10.00 A |
| 3600 W | 30.00 A | 15.65 A | 15.00 A |
Assumptions, limitations and practical tips
- The calculation assumes steady-state operation at a constant voltage. Motor starting (inrush) current can be 5–7 times the running current, so size circuits for inrush, not just the computed value.
- Nameplate power is often the output or rated power; the electrical input may be higher due to efficiency losses. Use the input power (or the nameplate current, if given) for circuit sizing.
- The voltages used must be RMS values, and the power factor must correspond to the actual load, not an assumed default. A small error in \(PF\) shifts the current proportionally — you can estimate it with the Power Factor Correction calculator.
- For long cable runs, the current also causes a voltage drop along the conductors; check it separately, and choose the conductor with the Wire Gauge calculator.
- Breaker suggestions are informational. Always verify conductor ampacity, equipment ratings and your local electrical code before installing anything.
Frequently Asked Questions
How do I convert watts to amps by hand?
Divide the power in watts by the voltage in volts. For a simple DC or resistive single-phase load: \( I = P / V \). So a 1500 W space heater on 120 V draws \( 1500 / 120 = 12.5 \) A. For AC with motors or other inductive loads, also divide by the power factor: \( I = P / (V \times PF) \), and for three-phase add the factor \(\sqrt{3}\) or 3 as described above.
How many amps is 1000 watts?
It depends entirely on the voltage. At 120 V a 1000 W resistive load draws about 8.33 A; at 230 V about 4.35 A; at 240 V about 4.17 A. The same 1000 W on a 12 V battery circuit pulls roughly 83 A. This is why the calculator always asks for voltage — there is no single amps value for a given wattage.
Why does a 1200 W motor draw more current than a 1200 W heater?
Because the motor's current is partly out of phase with the voltage. The nameplate watts are real (useful) power, but the wire must carry the apparent power \( S = P / PF \). A motor with a power factor of 0.8 draws \( 1 / 0.8 = 1.25 \) times more current than a heater with the same wattage at the same voltage. Always enter the actual power factor for inductive loads, not 1.0.
Should I enter 400 V or 230 V for a three-phase calculation?
It depends on which three-phase option you pick. If you choose line-to-line, enter 400 V and the calculator uses the \(\sqrt{3}\) factor. If you choose line-to-neutral, enter 230 V and it uses a factor of 3. Both paths give the same current — mixing them (400 V with the line-to-neutral option, or 230 V with line-to-line) makes the result wrong by a factor of \(\sqrt{3}\).
Why do I get fewer amps at higher voltage?
Current and voltage are inversely related when power is fixed: \( I = P / V \). Doubling the voltage halves the current for the same wattage. That is why high-power appliances (electric dryers, water heaters, EV chargers) run on 240 V rather than 120 V — lower current allows thinner, cheaper wire and smaller breakers.
Can I use this calculator for DC systems like 12 V or 24 V batteries?
Yes. Select DC as the circuit type, and the power factor is ignored (treated as 1). For example, a 300 W inverter load on a 12 V battery draws about 25 A from the battery — far more than the same load on mains voltage, which is why low-voltage DC wiring needs such thick conductors. Note the battery-side current, not the AC-side current, determines the DC wire and fuse size.
What is the most common mistake when converting watts to amps?
Using the nameplate output power instead of input power. A motor rated 1000 W mechanical output consumes more electrical power due to efficiency losses (an 85% efficient motor draws about 1176 W input). The second most common mistake is assuming a power factor of 1 for motor loads, which underestimates the current and can lead to undersized breakers and wiring. If the nameplate lists amps directly, use that value for circuit sizing.