Electrical Power Calculator
Calculate real, apparent and reactive power for single or three-phase supplies.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Electrical Power Calculator does
Electrical power splits into real power that does work, reactive power that circulates in inductive and capacitive loads, and apparent power that the supply must carry. Power factor is the ratio of real to apparent power, and three-phase supplies carry a √3 factor because the phases are 120° apart.
Formula
Single phase: P = V × I × PFThree phase: P = √3 × V × I × PFApparent power S = V × I (× √3 for three phase)Reactive power Q = S × √(1 − PF²)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Supply type | one of 2 options | Yes | — |
| Voltage | V | Yes | Accepts more than 0. |
| Current | A | Yes | Accepts more than 0. |
| Power factor | number | Yes | Accepts 0.01 or more, up to 1. |
| Efficiency | % | Optional | For motors — output power as a share of input. |
How to use it
- Choose Supply type.
- Enter Voltage, Current and Power factor.
- Optionally add Efficiency.
- Select Calculate.
Worked example
A 230 V single-phase load drawing 10 A at 0.95 power factor.
- Phase
- Single
- Voltage
- 230
- Current
- 10
- Power factor
- 0.95
Apparent 2,300 VA, real 2,185 W, reactive 718.2 VAR, phase angle 18.19°.
Reading the result
- Apparent power and real power are equal only when power factor is 1 — for any real-world inductive load, apparent power is larger, and the difference is reactive power, which does no useful work but still has to be supplied and accounted for when sizing equipment.
Common mistakes
- Sizing equipment from real power (P, in watts) alone when the relevant capacity limit is apparent power (S, in VA). A low-power-factor load can trip a breaker rated only from its wattage figure.
Frequently asked questions
What is the difference between kW and kVA?
kW is real power doing useful work. kVA is apparent power the supply must deliver. They are equal only at a power factor of 1.
Why does three-phase include √3?
Because the three phases peak 120° apart. Using line-to-line voltage, total power works out as √3 times the line voltage and current product.
Method and sources
Method. Standard AC and DC power relationships — P = V × I for DC, P = V × I × PF single phase, and P = √3 × V × I × PF for balanced three phase.
Applies to. None for the arithmetic; installation work is governed by national wiring regulations
Assumptions
- Sinusoidal supply, balanced load for the three-phase case, and the power factor entered is the true one.
Limitations
- Harmonic distortion from electronic loads raises apparent power above what these relationships predict, so a measured figure can exceed the calculation.
- An unbalanced three-phase load breaks the √3 relationship and must be computed per phase.
Sources
- The national wiring regulations applying where the installation is located — Varies by country. How any of this may be applied in practice. The relationships themselves are standard AC circuit theory.