How to Calculate Load in kW (Step-by-Step)

This guide shows you how to calculate electrical load in kilowatts (kW) for a circuit, appliance, or entire building. You’ll learn the basic formula, how to apply it to single-phase and three-phase systems, and how to convert amps or volts to kW. You’ll need basic information about the voltage, current, and power factor of the load.

Formula

The basic formula for electrical power in kilowatts is:

P (kW) = (V × I × PF × √3) / 1000 (for three-phase)

P (kW) = (V × I × PF) / 1000 (for single-phase)

Where:

• P = Power in kilowatts (kW)
• V = Voltage in volts (V)
• I = Current in amperes (A)
• PF = Power factor (a number between 0 and 1, often 0.8–0.95 for motors)
• √3 ≈ 1.732 (only for three-phase systems)

For DC circuits, PF = 1 and the √3 factor is not used.

Worked Example

Using the default values from the calculator: Voltage = 230 V, Current = 10 A, Power factor = 0.9, Single-phase.

Step 1: Multiply voltage by current: 230 × 10 = 2300 VA (volt-amperes).

Step 2: Multiply by power factor: 2300 × 0.9 = 2070 W (watts).

Step 3: Convert to kilowatts: 2070 / 1000 = 2.07 kW.

The calculator gives 2.07 kW, matching the manual calculation.

When to Use This Calculation

You use load calculation in kW when sizing wires, breakers, generators, or solar systems. It tells you how much real power a device consumes, which is what you pay for on your electricity bill (as opposed to apparent power in kVA). For resistive loads like heaters and incandescent bulbs, the power factor is 1. For inductive loads like motors and transformers, the power factor is less than 1.

How to Read the Result

The result is the real power consumed by the load. If you are sizing a generator, you need to account for starting surges (motors can draw 3–5 times their running current at startup). If you are sizing wires, you must also consider the current, not just the kW.

Common Mistakes

• Forgetting the power factor: Using PF = 1 for motors will underestimate the load.
• Mixing up single-phase and three-phase: Three-phase requires the √3 factor; using it for single-phase gives a wrong result.
• Using line-to-neutral voltage for three-phase: The formula assumes line-to-line voltage. If you have line-to-neutral voltage, multiply by 3 instead of √3.
• Not converting to kW: Forgetting to divide by 1000 leaves you in watts.

Tips

• Check the nameplate on the appliance for voltage, current, and power factor. If PF is not listed, use 0.8 for motors, 1 for resistive loads.
• For three-phase, measure voltage between two phases (line-to-line) and current in one phase.
• If you only know kVA, multiply by power factor to get kW: kW = kVA × PF.
• For a whole house, add up the kW of all major appliances, but apply a diversity factor (e.g., 0.5–0.8) because not all run at once.

Troubleshooting

If your calculated load seems too high or too low, check the following:

• Verify the voltage: In the US, residential is often 120/240 V; in Europe, 230 V. Use the actual measured voltage.
• Check the power factor: If the load is a motor, the PF may be lower than expected. Measure it with a power meter if possible.
• Confirm the phase: Is the circuit truly three-phase? Some appliances are single-phase but connected across two phases (which is still single-phase, not three-phase).
• Recheck the current: Use a clamp meter to measure the actual running current, not the nameplate rating, which may be the maximum.

Frequently Asked Questions

What is the difference between kW and kVA?

kW is real power (what does useful work). kVA is apparent power (volts × amps). The ratio is the power factor: kW = kVA × PF. For a PF of 0.8, 10 kVA equals 8 kW.

How do I calculate kW from amps?

Use the formula: kW = (V × I × PF) / 1000 for single-phase, or kW = (V × I × PF × 1.732) / 1000 for three-phase. You need to know the voltage and power factor.

What is a good power factor?

A power factor close to 1 is ideal. Most modern electronic devices have a PF near 0.9–0.99. Motors often have a PF of 0.7–0.9, especially when lightly loaded.

Can I calculate kW from kWh?

No, kWh is energy (power × time). To get kW, divide kWh by the number of hours: kW = kWh / hours. For example, 10 kWh over 5 hours = 2 kW average.

Why do I need the √3 for three-phase?

In a balanced three-phase system, the total power is the sum of the power in each phase. The √3 factor accounts for the phase shift between line-to-line voltage and phase current. It is derived from the geometry of three-phase systems.

What if my load is purely resistive?

Then the power factor is 1, so you can omit it. For example, a 230 V heater drawing 5 A uses 230 × 5 / 1000 = 1.15 kW.

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in v | Voltage | V | 230 | 0
in i | Current | A | 10 | 0
in pf | Power factor | - | 0.9 | 0
in ph | Phase | Single=1, Three=3 | 1 | 0
out kw | Power | kW | (v * i * pf * ph) / 1000 | 2