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Watts, Amps and Volts Calculator: DC, Single-Phase and Three-Phase
Choose the system type, pick which quantity to solve for and enter the other two. For AC, add the power factor. The calculator applies the right power formula and shows the working.
The power formulas for each system
Power, voltage and current are linked by one relationship, but the factor in front changes with the type of supply. In a DC circuit the power is the product of voltage and current. In a single-phase AC circuit, the power doing real work is reduced by the power factor, the cosine of the phase angle between voltage and current. In a balanced three-phase system with line-to-line voltage and line current, the factor √3 (about 1.732) appears as well.
| System | Power | Current |
|---|---|---|
| DC | V × I | P ÷ V |
| AC single-phase | V × I × PF | P ÷ (V × PF) |
| AC three-phase | √3 × V × I × PF | P ÷ (√3 × V × PF) |
Understanding power factor
Power factor describes how much of the current delivers useful power. Resistive loads such as heaters and incandescent lamps have a power factor near 1. Induction motors, transformers and some electronic supplies draw extra current that does no useful work, so their power factor is lower. At the same real power and voltage, a lower power factor means more current, which means thicker cable and larger protective devices.
Nameplates on motors often list power factor and efficiency separately. The wattage on a motor plate is usually the shaft output, so the electrical input is higher once efficiency is allowed for. If you only know the output, divide by the efficiency before using the formulas here.
Common mistakes
- Using the wrong voltage in three-phase work. The formula expects line-to-line voltage. Phase-to-neutral voltage is about 58 percent of it.
- Confusing watts and volt-amperes. Apparent power in kVA equals real power in kW divided by the power factor.
- Ignoring starting current. Motors can draw several times their running current for a moment, which these steady-state formulas do not show.
- Mixing prefixes. Convert kilowatts and milliamps before entering them, or read the result with that in mind.
Estimating, learning and safety
These results are estimates for learning and planning. Cable sizes, breaker ratings, derating for temperature or grouping, earthing and installation methods are governed by local electrical codes, and they do not follow from current alone. A qualified electrician or engineer must design and check any real installation, particularly at mains voltage, where mistakes can cause fires and fatal shocks. If a result is close to a limit, treat that as a reason to ask a professional rather than to trust a rounded number.
Worked example
A 10 kW three-phase motor load on a 400 V supply with a power factor of 0.8.
Inputs
| System | AC three-phase (line-to-line voltage) |
|---|---|
| Solve for | Current (amps) |
| Power | 10000 W |
| Voltage | 400 V |
| Power factor | 0.8 |
Result
For a three-phase AC system with power factor 0.8.
| Power | 10,000.00 W (10.000 kW) |
|---|---|
| Current | 18.042 A |
| Voltage | 400.00 V |
| Formula | I = P ÷ (√3 × V × PF) |
- For three-phase systems, enter the line-to-line voltage and the current in one line. Results are for steady-state estimates and ignore starting currents.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
How do I convert watts to amps?
For DC, divide watts by volts. For single-phase AC, divide watts by volts times the power factor. For three-phase, divide by √3 times line voltage times the power factor.
How many amps is 1000 watts at 120 volts?
On a DC or unity power factor circuit, 1000 watts at 120 volts draws 1000 divided by 120, or about 8.33 amps. A lower power factor would draw more current.
What is the √3 for in the three-phase formula?
It comes from the 120 degree phase shifts between the three lines. With line-to-line voltage and line current, total power from a balanced load includes the factor √3, about 1.732.
What power factor should I use if I do not know it?
Use 1 for resistive heating and look at the nameplate for motors. If it is unknown, test a lower value such as 0.8 to see how much the current rises.
Can I use this to choose a breaker or wire?
Use it only to understand the load. Breaker and wire selection depends on codes, temperature, cable length and installation method, so ask a qualified electrician.
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References and sources
- IEC and national electrical codes define apparent power, power factor and the conditions for selecting conductors and protective devices.