Watts / Volts / Amps Calculator
Calculate power, voltage or current for DC circuits, single-phase AC and three-phase AC. Enter any two known values, the third is calculated instantly. Includes power factor for AC loads.
Last updated: May 2026
Select circuit type, choose what to solve, then enter the two known values.
DC: P = V × I
DC vs single-phase vs three-phase power
The relationship between watts, volts and amps differs by circuit type. Getting the formula wrong, especially on three-phase, leads to undersized wiring, tripped breakers or incorrect energy cost estimates. If you are still mapping out a circuit, the wider Electronics calculators hub covers the resistor, voltage and reactance steps that usually surround a power calculation.
Formula reference
| Circuit | Power formula | Notes |
|---|---|---|
| DC | P = V × I | No power factor. Used for battery systems, 12/24 V loads. |
| AC single-phase | P = V × I × PF | PF = 1.0 for pure resistive loads (heaters, incandescents). Motors, switching PSUs typically 0.6-0.95. |
| AC three-phase | P = √3 × VL × IL × PF | VL = line voltage (e.g. 400 V). √3 ≈ 1.732. Used for industrial motors, EV fast chargers. |
Typical use cases
- Sizing a fuse or circuit breaker for a known load
- Estimating current draw from a rated power figure on a nameplate
- Checking if a supply is large enough for a motor or inverter
- Converting between apparent power (VA) and real power (W) using power factor
Sizing the supply once the circuit current is known
By the time you reach this step you have worked out the current each branch of your circuit draws. Totalling that load and converting it to a supply requirement is what this page does. Here is where the DC build chain stands:
- Start with the base relationships. Voltage, current, resistance and power are all linked. The Ohm's Law calculator pins down those four numbers from any two you already know.
- Protect each LED with a series resistor. Forward voltage, supply rail and target current determine the limiting resistor. The LED resistor calculator works it out and rounds to a real part value.
- Scale a signal or set a reference with a divider. The voltage divider calculator gives the output voltage and the quiescent current the divider burns.
- Decode the parts from the bin. Band markings and printed codes are resolved by the resistor color code and the capacitor code calculator.
- You are here: size the supply for the load. Add up the power each branch consumes, find the total current at your rail voltage, and confirm the supply can deliver it. Add headroom: a rail run continuously at its rated limit runs hot and ages faster than one with margin to spare.
- Check the wire on a long run. Current through cable resistance drops voltage and wastes power. The voltage drop calculator sizes cable for LED strips, solar and vehicle runs.
- Estimate battery runtime. Total load current against pack capacity gives run time. The battery life calculator applies a real-world efficiency factor.
If your load is AC rather than steady DC, the impedance tools take over: the impedance calculator, capacitor reactance and inductor reactance handle the reactive parts of the circuit.
I always size a supply with headroom, at least 20 to 25 percent above the calculated continuous load. A converter or PSU run flat out gets noticeably warm, its efficiency curve drops off, and the capacitors age faster. That margin costs almost nothing and buys years of reliable running, which is why I never spec a rail right at the number this calculator returns.
Frequently Asked Questions
What power factor should I use for a motor?
Induction motors typically have PF between 0.7 and 0.9 at full load, and significantly lower at partial load. The nameplate usually lists both kW and kVA, divide the two to get PF. For rough estimation when the datasheet is unavailable, use 0.85 for motors 1 kW and above, 0.75 for smaller motors.
Why is three-phase more efficient than single-phase?
Three-phase delivers the same total power at a lower current per conductor because the load is distributed across three phases. A 10 kW load on 230 V single-phase pulls 43.5 A per conductor. The same load on 400 V three-phase at PF 0.9 pulls only 16.1 A per conductor, about 37% of the single-phase current. This reduces conductor size and resistive losses.
How much headroom should I add when sizing a DC supply for a continuous load?
A common rule for continuous DC loads is to choose a supply rated at least 20 to 25 percent above the calculated peak draw. Running a converter or linear supply at its rated maximum raises its internal temperature, drops efficiency and shortens the life of its capacitors and magnetics. For example, a circuit that draws 8 A continuously is better served by a 10 A supply than a 9 A one. If the load includes inrush spikes (motors, large capacitor banks) the peak current can be several times the running current, so check the supply's peak and sustained current ratings separately.
How do I check the current draw of a 400 V three-phase motor?
Use I = P ÷ (√3 × V × PF). For a 7.5 kW motor at 400 V and PF 0.87: I = 7500 ÷ (1.732 × 400 × 0.87) = 12.45 A. The three-phase mode in this calculator handles the √3 factor automatically, enter the kW figure in the Watts field (as watts, e.g. 7500), set voltage to 400 and power factor to 0.87.
Methodology and sources
This tool solves the power relationship between watts, volts and amps for DC, single-phase AC and three-phase AC circuits. You enter any two known values and it returns the third using the standard power equation for the selected circuit type.
- Method: DC uses P = V × I. Single-phase AC uses P = V × I × PF. Three-phase AC uses P = √3 × VL × IL × PF, where √3 ≈ 1.732 and VL is the line-to-line voltage. To solve for voltage or current the equation is rearranged, for example I = P / (√3 × V × PF).
- Standards and sources: Standard electrical physics, the real-power equations for DC and AC circuits, including the √3 line-power relation for balanced three-phase systems. Nominal voltages such as 230 V single-phase and 400 V three-phase follow IEC 60038. The tool does not apply any wiring, ampacity or breaker-sizing standard.
- Assumptions and limits: Three-phase results assume a balanced load. Power factor is treated as a single value between 0.01 and 1.0 (set to 1.0 for DC and pure resistive loads). The tool reports the electrical power, voltage or current only, it does not size conductors, fuses or breakers and does not apply any de-rating for heat, grouping or continuous-load margins.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning aid, not a substitute for a qualified electrician or your local wiring and building code. Always size fuses, breakers and conductors per the applicable code and have safety-critical work verified by a qualified professional.
Next step in this workflow
Power calculated: now design a voltage divider for your supply.