Electrical power calculator
Active and apparent power for single and three phase installations.
Calculator inputs
Results
Enter your values and press “Calculate” to see the result.
In short
- What it calculates
- Active and apparent power for single and three phase installations.
- Formula used
Single phase: P = V × I × cos φ- Example
- 230 V, 16 A and power factor 0.9, single phase:
Electrical power calculator
In a real installation useful power is not simply voltage times current: the power factor and the system type change the result.
This calculator covers both cases and also reports apparent power, which is what sizes the cabling.
How it works
In single phase you multiply voltage, current and power factor. In three phase a factor of root three is added, because the three phases are 120 degrees apart.
Formula
Single phase: P = V × I × cos φ
Three phase: P = √3 × V × I × cos φ
Apparent power: S = V × I (or √3 × V × I)
1 metric hp = 735.49875 W
Worked example
230 V, 16 A and power factor 0.9, single phase:
P = 230 × 16 × 0.9 = 3,312 W
S = 230 × 16 = 3,680 VA
That is 4.5 metric horsepower.
Explanation
Active, apparent and reactive power
Active power, in watts, is what actually does work. Apparent power, in volt-amperes, is what flows through the cable. In loads with motors or transformers, part of the energy goes back and forth without doing useful work: that is reactive power. The cable has to carry the apparent power, even though only the active is billed.
Power factor
It is the ratio of active to apparent. A pure resistance has a factor of one, and a motor can drop to 0.7. A low factor forces you to oversize the installation and, on industrial tariffs, triggers penalties, which is why factories install capacitor banks to correct it.
Single phase and three phase
In three-phase systems a square root of three factor appears in the formula, and that is where most mistakes are made. The same power is carried with less current per conductor, which is why industry uses three-phase.
Sizing the cable properly
Choosing a cross-section by real current rather than nominal power prevents overheating. An undersized cable does not fail on day one: it degrades, its resistance rises and it ends up a safety problem.
Frequently asked questions
What is power factor?
The share of energy that actually does work. Motors and transformers lower it; resistive loads and incandescent bulbs keep it at 1.
Why does apparent power matter?
Because cabling and protective devices are sized on total current, not just on useful power.
Why does my installation need more amps than expected?
Because power factor reduces useful power against what actually flows. At a factor of 0.8, the required current is twenty-five per cent higher.
Why does industry use three-phase power?
Because it carries the same power with less current per conductor, allowing thinner cables and more efficient motors.
Need to calculate something else?
These tools are often used alongside this calculator.
Ohm’s law
Enter two of voltage, current or resistance and get the third.
Electricity use
What an appliance costs per month and per year based on hours of use.