Electrical Load Calculator

How to use the Electrical Load Calculator

Select the type of electrical supply and enter the supply voltage. Add each load to the worksheet using its rated power, quantity and power factor. The utilisation percentage can be used when only part of the connected load is expected to operate at the same time.

For example, if four identical 500 W loads are installed but normally only three are expected to operate together, a utilisation value of 75% can be used as a simple calculation assumption.

Electrical load calculation formula

For a single-phase AC circuit, active electrical power can be expressed as:

For a balanced three-phase AC system:

Apparent power is measured in volt-amperes or kilovolt-amperes. Power factor is the ratio between active power and apparent power.

Watts, VA, kW and kVA

Watts and kilowatts describe active power — the portion of electrical power converted into useful work, heat, light or mechanical output. VA and kVA describe apparent power, which also accounts for the effect of power factor in AC circuits.

When a load is entered in watts or kilowatts, this calculator uses the entered power factor to estimate its apparent and reactive power. When a load is entered directly in VA or kVA, the entered value is treated as apparent power.

What is power factor?

Power factor describes the relationship between active power measured in watts and apparent power measured in volt-amperes. A power factor of 1.00 means that active power and apparent power are equal.

Many AC loads have a power factor below 1. Motors, transformers and other equipment can require both active and reactive power. For a given amount of active power, a lower power factor generally means that more current is required from the electrical supply.

When combining several AC loads, this calculator calculates the active and reactive components of each entered load before determining the overall apparent power and power factor.

For definitions of electrical SI quantities and units, including amperes, volts and watts, see the NIST guidance on SI units for electric current .

What does the utilisation percentage mean?

The utilisation percentage allows you to make a simple assumption about how much of an individual connected load operates simultaneously. A value of 100% includes the entire load. A value of 50% includes half of it in the calculated total.

How three-phase electrical load is calculated

For balanced three-phase loads, the calculator uses the line-to-line voltage and the square root of three in the current calculation. The result assumes approximately balanced loading between phases.

Electrical load calculation example

Suppose a single-phase 230 V supply carries ten 100 W loads at a power factor of 0.95 and all ten are expected to operate. The active load is 1,000 W. The corresponding apparent load is approximately 1,053 VA, giving a supply current of approximately 4.58 A.

Limitations

This calculator performs general electrical power arithmetic. It does not determine cable size, breaker size, conductor ampacity, fault current, voltage drop, phase balancing, starting current or regulatory demand factors.

Electrical installation requirements vary by country, application and applicable electrical code. For design, installation or safety decisions, use the relevant electrical standard and appropriately qualified professional guidance.

Frequently asked questions