How to Calculate Electrical Power

Electrical power can be calculated from voltage and current, but the exact formula depends on whether the electrical system is DC, single-phase AC, or three-phase AC.

The Electrical Power Calculator above allows you to choose the electrical system and calculate real power in watts and kilowatts. For AC systems, it also calculates apparent power in VA and kVA.

DC Power Formula

For a DC electrical circuit:

Watts = Volts × Amps

The formula can also be written as:

P = V × I

Where:

P = power in watts
V = voltage in volts
I = current in amperes

DC Power Example

Suppose a DC device operates at 24 volts and draws 10 amps.

Watts = 24 × 10

Watts = 240 W

In kilowatts:

240 ÷ 1000 = 0.24 kW

Therefore, the DC load uses 240 watts or 0.24 kW.

Single-Phase AC Power Formula

For a single-phase AC electrical system:

Watts = Volts × Amps × Power Factor

For kilowatts:

kW = (Volts × Amps × Power Factor) ÷ 1000

The apparent power formula is:

VA = Volts × Amps

And:

kVA = (Volts × Amps) ÷ 1000

Single-Phase Power Example

Suppose a single-phase load operates at 230 volts, draws 10 amps, and has a power factor of 0.8.

Apparent power:

VA = 230 × 10 = 2,300 VA

kVA = 2.3 kVA

Real power:

Watts = 230 × 10 × 0.8

Watts = 1,840 W

kW = 1.84 kW

For a dedicated single-phase tool, use our

Single-Phase Power Calculator
.

Three-Phase AC Power Formula

For a balanced three-phase AC electrical system using line-to-line voltage and line current:

Watts = √3 × Volts × Amps × Power Factor

For kilowatts:

kW = (√3 × Volts × Amps × Power Factor) ÷ 1000

Apparent power is:

VA = √3 × Volts × Amps

And:

kVA = (√3 × Volts × Amps) ÷ 1000

The value of √3 is approximately 1.732.

Three-Phase Power Example

Suppose a balanced three-phase electrical load operates at 400 volts line-to-line, draws 10 amps of line current, and has a power factor of 0.8.

Apparent power:

VA = 1.732 × 400 × 10

VA = approximately 6,928 VA

kVA = approximately 6.93 kVA

Real power:

Watts = 1.732 × 400 × 10 × 0.8

Watts = approximately 5,543 W

kW = approximately 5.54 kW

For a dedicated three-phase calculation, use our

Three-Phase Power Calculator
.

Electrical Power Formula Reference

What Is Electrical Power?

Electrical power is the rate at which electrical energy is transferred or converted into another form of energy.

Power can be used to produce heat, light, movement, cooling, sound, computing output, or other useful work.

The standard unit of real electrical power is the watt.

One kilowatt is equal to 1,000 watts.

What Is Voltage?

Voltage is the electrical potential difference between two points in a circuit and is measured in volts.

Voltage is one of the main values used to calculate electrical power.

What Is Current?

Electrical current is the flow of electric charge through a circuit and is measured in amperes, commonly called amps.

Electrical power generally increases as current increases when other conditions remain the same.

What Is Power Factor?

Power factor represents the relationship between real power and apparent power in an AC electrical system.

It is normally expressed as a value between 0 and 1.

For example:

0.8 power factor = 80%

0.9 power factor = 90%

1.0 power factor = unity power factor

Power factor is needed when calculating AC real power from voltage and current.

If real and apparent power are already known, use our

Power Factor Calculator
.

Why Does DC Not Require Power Factor?

The basic DC power calculation uses voltage and current directly:

Watts = Volts × Amps

The AC power factor term used to distinguish real power from apparent power is therefore not included in the DC mode of this calculator.

What Is Real Power?

Real power is the electrical power that performs useful work and is measured in watts or kilowatts.

Examples include power converted into heat by a heater, mechanical output from a motor, light from lighting equipment, or useful electronic operation in computers and other devices.

What Is Apparent Power?

Apparent power is used in AC systems and represents the product of voltage and current while accounting for the phase relationship through the distinction between real and apparent power.

It is measured in volt-amperes or kilovolt-amperes.

UPS systems, generators, transformers, inverters, and other AC electrical equipment may be rated in VA or kVA.

Watts vs VA

Watts represent real power, while VA represent apparent power.

Their relationship is:

Watts = VA × Power Factor

At a power factor of 1, watts and VA have the same numerical value.

At a power factor below 1, watts are lower than VA.

kW vs kVA

Kilowatts and kilovolt-amperes represent the same real-power and apparent-power relationship on a larger scale.

The relationship is:

kW = kVA × Power Factor

For example, 10 kVA at a power factor of 0.8 corresponds to:

10 × 0.8 = 8 kW

DC vs Single-Phase vs Three-Phase Power

DC, single-phase AC, and three-phase AC systems require different formulas because their voltage and current relationships are different.

DC uses:

P = V × I

Single-phase AC uses:

P = V × I × PF

Balanced three-phase AC using line-to-line voltage uses:

P = √3 × V × I × PF

Choosing the correct electrical system is therefore important before calculating power.

Common Uses of an Electrical Power Calculator

An electrical power calculator can be used for residential, commercial, industrial, IT, electrical distribution, and equipment planning applications.

Common examples include computers, servers, UPS systems, generators, transformers, motors, pumps, lighting, appliances, power supplies, batteries, DC equipment, air-conditioning systems, audio and video equipment, machinery, and electrical panels.

The calculator is especially useful when voltage and current are known and you need to estimate watts, kilowatts, VA, or kVA.

Frequently Asked Questions

What is the basic electrical power formula?

For DC:

Watts = Volts × Amps

AC calculations may also require power factor and, for balanced three-phase systems, the √3 factor.

How do I calculate watts from volts and amps?

For DC:

Watts = Volts × Amps

For single-phase AC:

Watts = Volts × Amps × Power Factor

How do I calculate three-phase watts?

For a balanced three-phase system using line-to-line voltage:

Watts = √3 × Volts × Amps × Power Factor

How do I convert watts to kilowatts?

Divide watts by 1,000.

kW = Watts ÷ 1000

How many watts is 24 volts at 10 amps DC?

24 × 10 = 240 watts.

How many watts is 12 volts at 10 amps?

For a DC circuit:

12 × 10 = 120 watts.

How many watts is 230 volts at 10 amps?

For single-phase AC, the result depends on power factor.

At a power factor of 1:

230 × 10 × 1 = 2,300 watts.

At a power factor of 0.8:

230 × 10 × 0.8 = 1,840 watts.

How many kW is 400 volts three-phase at 10 amps?

At a power factor of 0.8:

1.732 × 400 × 10 × 0.8 ÷ 1000 = approximately 5.54 kW.

Do I need power factor for DC?

No. The basic DC power formula is watts = volts × amps.

Do I need power factor for single-phase AC?

Yes, when calculating real power in watts or kW from voltage and current.

Do I need power factor for three-phase AC?

Yes, when calculating real power from voltage and current.

Do I need power factor to calculate VA or kVA?

No. Apparent power can be calculated directly from voltage and current.

Why does three-phase power use 1.732?

The number 1.732 is an approximation of √3. It comes from the mathematical relationship between the phases in a balanced three-phase system.

What voltage should I use for three-phase calculations?

This calculator uses line-to-line voltage for its three-phase mode.

Can I use this calculator for an unbalanced three-phase system?

The three-phase mode is intended for balanced systems. Unbalanced loads may require measurements or calculations for each individual phase.

Can I use this calculator for a UPS?

Yes. It can help estimate load power, but final UPS selection should use both the manufacturer’s watt and VA ratings and should also consider runtime, overload capability, redundancy, future expansion, and other system requirements.

Can I use this calculator for a generator?

It can help estimate electrical load power. Final generator sizing may also need to consider motor starting currents, nonlinear loads, load steps, harmonics, environmental conditions, future loads, and manufacturer recommendations.

Can I use this calculator to size electrical wiring?

No. Calculating electrical power alone is not enough to determine conductor size.

Wire sizing can depend on current, conductor material, insulation rating, installation method, temperature, voltage drop, continuous load requirements, protective devices, derating factors, and applicable electrical codes.

Electrical Calculation Disclaimer

This Electrical Power Calculator is provided for informational, educational, and preliminary planning purposes. Calculated values may be rounded.

DC mode uses the basic voltage-current power relationship. Single-phase AC mode uses voltage, current, and power factor. Three-phase AC mode assumes a balanced system and uses line-to-line voltage and line current.

Actual electrical conditions may vary because of voltage variation, load imbalance, power factor changes, harmonics, waveform distortion, equipment efficiency, starting current, measurement accuracy, operating conditions, and other factors.

Electrical equipment should be selected using manufacturer ratings and specifications. Electrical installation design may also require consideration of conductor ampacity, voltage drop, continuous loads, protective devices, fault current, derating factors, installation method, temperature, and applicable electrical codes.

Consult a qualified electrical professional when making final electrical installation, wiring, conductor sizing, protective device, equipment selection, generator sizing, UPS sizing, or other electrical system decisions.